diff --git a/reactos/dll/win32/user32/windows/message.c b/reactos/dll/win32/user32/windows/message.c
index a7921e5df35..add13934440 100644
--- a/reactos/dll/win32/user32/windows/message.c
+++ b/reactos/dll/win32/user32/windows/message.c
@@ -1014,8 +1014,9 @@ CallWindowProcA(WNDPROC lpPrevWndFunc,
 {
   WNDPROC_INFO wpInfo;
 
+  /* FIXME - can the first parameter be NULL? */
   if (lpPrevWndFunc == NULL)
-    lpPrevWndFunc = (WNDPROC)NtUserGetWindowLong(hWnd, GWLP_WNDPROC, FALSE);
+    lpPrevWndFunc = (WNDPROC)NtUserGetWindowLong(hWnd, GWLP_WNDPROC, TRUE);
 
   if (!NtUserDereferenceWndProcHandle((HANDLE)lpPrevWndFunc,
                                       &wpInfo))
@@ -1042,6 +1043,10 @@ CallWindowProcW(WNDPROC lpPrevWndFunc,
 {
   WNDPROC_INFO wpInfo;
 
+  /* FIXME - can the first parameter be NULL? */
+  if (lpPrevWndFunc == NULL)
+    lpPrevWndFunc = (WNDPROC)NtUserGetWindowLong(hWnd, GWLP_WNDPROC, FALSE);
+
   if (!NtUserDereferenceWndProcHandle((HANDLE)lpPrevWndFunc,
                                       &wpInfo))
     {
diff --git a/reactos/include/reactos/win32k/ntuser.h b/reactos/include/reactos/win32k/ntuser.h
index 035ee7a985e..4e328ff818e 100644
--- a/reactos/include/reactos/win32k/ntuser.h
+++ b/reactos/include/reactos/win32k/ntuser.h
@@ -14,7 +14,7 @@ typedef struct _CALLPROC
 {
     struct _W32PROCESSINFO *pi;
     WNDPROC WndProc;
-    BOOL Unicode : 1;
+    UINT Unicode : 1;
 } CALLPROC, *PCALLPROC;
 
 typedef struct _WINDOWCLASS
@@ -46,10 +46,10 @@ typedef struct _WINDOWCLASS
 
     ULONG_PTR ClassExtraDataOffset;
 
-    BOOL Destroying : 1;
-    BOOL Unicode : 1;
-    BOOL System : 1;
-    BOOL Global : 1;
+    UINT Destroying : 1;
+    UINT Unicode : 1;
+    UINT System : 1;
+    UINT Global : 1;
 } WINDOWCLASS, *PWINDOWCLASS;
 
 typedef struct _W32PROCESSINFO
diff --git a/reactos/subsystems/win32/win32k/include/ssec.h b/reactos/subsystems/win32/win32k/include/ssec.h
deleted file mode 100644
index e69de29bb2d..00000000000
diff --git a/reactos/subsystems/win32/win32k/include/window.h b/reactos/subsystems/win32/win32k/include/window.h
index ae46e6998d2..74be462f6b2 100644
--- a/reactos/subsystems/win32/win32k/include/window.h
+++ b/reactos/subsystems/win32/win32k/include/window.h
@@ -37,6 +37,9 @@ typedef struct _WINDOW_OBJECT
     /* Extra Wnd proc (windows of system classes) */
     WNDPROC WndProcExtra;
   };
+  /* Pointer to another call procedure handle (used for returning the previous
+     window proc in SetWindowLongPtr) */
+  PCALLPROC CallProc2;
   /* Indicates whether the window is derived from a system class */
   BOOL IsSystem;
   /* Pointer to the window class. */
diff --git a/reactos/subsystems/win32/win32k/ntuser/class.c b/reactos/subsystems/win32/win32k/ntuser/class.c
index 25ffd83e49f..3fe2fbe5707 100644
--- a/reactos/subsystems/win32/win32k/ntuser/class.c
+++ b/reactos/subsystems/win32/win32k/ntuser/class.c
@@ -89,7 +89,7 @@ CreateCallProc(IN PDESKTOP Desktop,
     {
         NewCallProc->pi = pi;
         NewCallProc->WndProc = WndProc;
-        NewCallProc->Unicode = Unicode;
+        NewCallProc->Unicode = Unicode != FALSE;
     }
 
     return NewCallProc;
@@ -109,13 +109,11 @@ UserGetCallProcInfo(IN HANDLE hCallProc,
                              otCallProc);
     if (CallProc == NULL)
     {
-        SetLastWin32Error(ERROR_INVALID_HANDLE);
         return FALSE;
     }
 
     if (CallProc->pi != GetW32ProcessInfo())
     {
-        SetLastWin32Error(ERROR_ACCESS_DENIED);
         return FALSE;
     }
 
@@ -322,12 +320,69 @@ IntSetClassAtom(IN OUT PWINDOWCLASS Class,
     return TRUE;
 }
 
+static WNDPROC
+IntGetClassWndProc(IN PWINDOWCLASS Class,
+                   IN PW32PROCESSINFO pi,
+                   IN BOOL Ansi)
+{
+    /* FIXME - assert for exclusive lock! */
+
+    if (Class->System)
+    {
+        return (Ansi ? Class->WndProcExtra : Class->WndProc);
+    }
+    else
+    {
+        if (!Ansi == Class->Unicode)
+        {
+            return Class->WndProc;
+        }
+        else
+        {
+            if (Class->CallProc != NULL)
+            {
+                return (WNDPROC)ObmObjectToHandle(Class->CallProc);
+            }
+            else
+            {
+                PCALLPROC NewCallProc, CallProc;
+
+                if (pi == NULL)
+                    return NULL;
+
+                /* NOTE: use the interlocked functions, as this operation may be done even
+                         when only the shared lock is held! */
+                NewCallProc = CreateCallProc(Class->Desktop,
+                                             Class->WndProc,
+                                             Class->Unicode,
+                                             pi);
+                if (NewCallProc == NULL)
+                {
+                    SetLastWin32Error(ERROR_NOT_ENOUGH_MEMORY);
+                    return NULL;
+                }
+
+                CallProc = InterlockedCompareExchangePointer(&Class->CallProc,
+                                                             NewCallProc,
+                                                             NULL);
+                if (CallProc != NULL)
+                {
+                    DestroyCallProc(Class->Desktop,
+                                    NewCallProc);
+                }
+
+                return (WNDPROC)ObmObjectToHandle((CallProc == NULL ? NewCallProc : CallProc));
+            }
+        }
+    }
+}
+
 static WNDPROC
 IntSetClassWndProc(IN OUT PWINDOWCLASS Class,
                    IN WNDPROC WndProc,
                    IN BOOL Ansi)
 {
-    WNDPROC Ret = Class->WndProc;
+    WNDPROC Ret;
 
     if (Class->System)
     {
@@ -336,6 +391,14 @@ IntSetClassWndProc(IN OUT PWINDOWCLASS Class,
         return NULL;
     }
 
+    Ret = IntGetClassWndProc(Class,
+                             GetW32ProcessInfo(),
+                             Ansi);
+    if (Ret == NULL)
+    {
+        return NULL;
+    }
+
     /* update the base class first */
     Class = Class->Base;
 
@@ -733,61 +796,6 @@ IntFindClass(IN RTL_ATOM Atom,
     return Class;
 }
 
-static WNDPROC
-IntGetClassWndProc(IN PWINDOWCLASS Class,
-                   IN PW32PROCESSINFO pi,
-                   IN BOOL Ansi)
-{
-    if (Class->System)
-    {
-        return (Ansi ? Class->WndProcExtra : Class->WndProc);
-    }
-    else
-    {
-        if (!Ansi == Class->Unicode)
-        {
-            return Class->WndProc;
-        }
-        else
-        {
-            if (Class->CallProc != NULL)
-            {
-                return (WNDPROC)ObmObjectToHandle(Class->CallProc);
-            }
-            else
-            {
-                PCALLPROC NewCallProc, CallProc;
-
-                if (pi == NULL)
-                    return NULL;
-
-                /* NOTE: use the interlocked functions, as this operation may be done even
-                         when only the shared lock is held! */
-                NewCallProc = CreateCallProc(Class->Desktop,
-                                             Class->WndProc,
-                                             Class->Unicode,
-                                             pi);
-                if (NewCallProc == NULL)
-                {
-                    SetLastWin32Error(ERROR_NOT_ENOUGH_MEMORY);
-                    return NULL;
-                }
-
-                CallProc = InterlockedCompareExchangePointer(&Class->CallProc,
-                                                             NewCallProc,
-                                                             NULL);
-                if (CallProc != NULL)
-                {
-                    DestroyCallProc(Class->Desktop,
-                                    NewCallProc);
-                }
-
-                return (WNDPROC)ObmObjectToHandle((CallProc == NULL ? NewCallProc : CallProc));
-            }
-        }
-    }
-}
-
 RTL_ATOM
 IntGetClassAtom(IN PUNICODE_STRING ClassName,
                 IN HINSTANCE hInstance  OPTIONAL,
@@ -1760,22 +1768,9 @@ UserGetClassInfo(IN PWINDOWCLASS Class,
 {
     lpwcx->style = Class->Style;
 
-    if (Class->System)
-    {
-        lpwcx->lpfnWndProc = (!Ansi ? Class->WndProc : Class->WndProcExtra);
-    }
-    else
-    {
-        if (!Ansi == Class->Unicode)
-        {
-            lpwcx->lpfnWndProc = Class->WndProc;
-        }
-        else
-        {
-            /* FIXME - return callproc handle or function pointer? */
-            lpwcx->lpfnWndProc = Class->CallProc->WndProc;
-        }
-    }
+    lpwcx->lpfnWndProc = IntGetClassWndProc(Class,
+                                            GetW32ProcessInfo(),
+                                            Ansi);
 
     lpwcx->cbClsExtra = Class->ClsExtra;
     lpwcx->cbWndExtra = Class->WndExtra;
@@ -1917,7 +1912,14 @@ NtUserGetClassLong(IN HWND hWnd,
     PWINDOW_OBJECT Window;
     ULONG_PTR Ret = 0;
 
-    UserEnterShared();
+    if (Offset != GCLP_WNDPROC)
+    {
+        UserEnterShared();
+    }
+    else
+    {
+        UserEnterExclusive();
+    }
 
     Window = UserGetWindowObject(hWnd);
     if (Window != NULL)
@@ -2090,7 +2092,9 @@ NtUserGetClassInfo(
     PW32PROCESSINFO pi;
     BOOL Ret = FALSE;
 
-    UserEnterShared();
+    /* NOTE: need exclusive lock because getting the wndproc might require the
+             creation of a call procedure handle */
+    UserEnterExclusive();
 
     pi = GetW32ProcessInfo();
     if (pi == NULL)
diff --git a/reactos/subsystems/win32/win32k/ntuser/message.c b/reactos/subsystems/win32/win32k/ntuser/message.c
index 4bfeb369949..b6005ca0e63 100644
--- a/reactos/subsystems/win32/win32k/ntuser/message.c
+++ b/reactos/subsystems/win32/win32k/ntuser/message.c
@@ -380,7 +380,10 @@ NtUserDispatchMessage(PNTUSERDISPATCHMESSAGEINFO UnsafeMsgInfo)
             Result = 0;
 
             MsgInfo.Ansi = !Window->Unicode;
-            MsgInfo.Proc = Window->WndProc;
+            if (Window->IsSystem)
+                MsgInfo.Proc = (Window->Unicode ? Window->WndProc : Window->WndProcExtra);
+            else
+                MsgInfo.Proc = Window->WndProc;
          }
       }
    }
@@ -1563,7 +1566,10 @@ co_IntDoSendMessage(HWND hWnd,
       }
 
       Info.Ansi = !Window->Unicode;
-      Info.Proc = Window->WndProc;
+      if (Window->IsSystem)
+          Info.Proc = (Window->Unicode ? Window->WndProc : Window->WndProcExtra);
+      else
+          Info.Proc = Window->WndProc;
    }
    else
    {
diff --git a/reactos/subsystems/win32/win32k/ntuser/window.c b/reactos/subsystems/win32/win32k/ntuser/window.c
index e886057ade5..bc095aa54bd 100644
--- a/reactos/subsystems/win32/win32k/ntuser/window.c
+++ b/reactos/subsystems/win32/win32k/ntuser/window.c
@@ -440,6 +440,13 @@ static LRESULT co_UserFreeWindow(PWINDOW_OBJECT Window,
                        Window->CallProc);
    }
 
+   if (Window->CallProc2 != NULL)
+   {
+       DbgPrint("!!!!! Destroy call proc 0x%p\n", ObmObjectToHandle(Window->CallProc2));
+       DestroyCallProc(Window->ti->Desktop,
+                       Window->CallProc2);
+   }
+
    /* dereference the class */
    IntDereferenceClass(Window->Class,
                        Window->ti->Desktop,
@@ -1572,17 +1579,10 @@ co_IntCreateWindowEx(DWORD dwExStyle,
    Window->IsSystem = Class->System;
    if (Class->System)
    {
-       Window->Unicode = bUnicodeWindow;
-       if (bUnicodeWindow)
-       {
-           Window->WndProc = Class->WndProc;
-           Window->WndProcExtra = Class->WndProcExtra;
-       }
-       else
-       {
-           Window->WndProc = Class->WndProcExtra;
-           Window->WndProcExtra = Class->WndProc;
-       }
+       /* NOTE: Always create a unicode window for system classes! */
+       Window->Unicode = TRUE;
+       Window->WndProc = Class->WndProc;
+       Window->WndProcExtra = Class->WndProcExtra;
    }
    else
    {
@@ -3422,7 +3422,93 @@ CLEANUP:
    END_CLEANUP;
 }
 
+static WNDPROC
+IntSetWindowProc(PWINDOW_OBJECT Window,
+                 WNDPROC NewWndProc,
+                 BOOL Ansi)
+{
+    WNDPROC Ret;
 
+    /* attempt to get the previous window proc */
+    if (Window->IsSystem)
+    {
+        Ret = (Ansi ? Window->WndProcExtra : Window->WndProc);
+    }
+    else
+    {
+        if (!Ansi == Window->Unicode)
+        {
+            Ret = Window->WndProc;
+        }
+        else
+        {
+            /* allocate or update an existing call procedure handle to return
+               the old window proc */
+            if (Window->CallProc2 != NULL)
+            {
+                Window->CallProc2->WndProc = Window->WndProc;
+                Window->CallProc2->Unicode = Window->Unicode;
+            }
+            else
+            {
+                Window->CallProc2 = CreateCallProc(Window->ti->Desktop,
+                                                   Window->WndProc,
+                                                   Window->Unicode,
+                                                   Window->ti->kpi);
+                if (Window->CallProc2 == NULL)
+                {
+                    SetLastWin32Error(ERROR_NOT_ENOUGH_MEMORY);
+                    return NULL;
+                }
+            }
+
+            Ret = (WNDPROC)ObmObjectToHandle(Window->CallProc2);
+            DbgPrint("!!!!!!!! Returning handle 0x%p\n", Ret);
+        }
+    }
+
+    if (Window->Class->System)
+    {
+        BOOL SysWnd = Window->IsSystem;
+
+        /* check if the new procedure matches with the one in the
+           window class. If so, we need to restore both procedures! */
+        Window->IsSystem = (NewWndProc == Window->Class->WndProc ||
+                            NewWndProc == Window->Class->WndProcExtra);
+
+        if (Window->IsSystem != SysWnd)
+        {
+            if (!Window->IsSystem && Window->CallProc != NULL)
+            {
+                /* destroy the callproc, we don't need it anymore */
+                DestroyCallProc(Window->ti->Desktop,
+                                Window->CallProc);
+                Window->CallProc = NULL;
+            }
+        }
+
+        if (Window->IsSystem)
+        {
+            Window->WndProc = Window->Class->WndProc;
+            Window->WndProcExtra = Window->Class->WndProcExtra;
+            Window->Unicode = !Ansi;
+            return Ret;
+        }
+    }
+
+    ASSERT(!Window->IsSystem);
+
+    /* update the window procedure */
+    Window->WndProc = NewWndProc;
+    if (Window->CallProc != NULL)
+    {
+        Window->CallProc->WndProc = NewWndProc;
+        Window->CallProc->Unicode = !Ansi;
+    }
+    Window->Unicode = !Ansi;
+
+    return Ret;
+}
 
 
 LONG FASTCALL
@@ -3490,24 +3576,9 @@ co_UserSetWindowLong(HWND hWnd, DWORD Index, LONG NewValue, BOOL Ansi)
          case GWL_WNDPROC:
          {
             /* FIXME: should check if window belongs to current process */
-            if (Window->IsSystem)
-            {
-               /* the user changes the window procedure, the window is no longer
-                  directly derived from the system class, because it no longer
-                  uses independent window procedures for ansi and unicode */
-               Window->IsSystem = FALSE;
-               Window->CallProc = NULL;
-            }
-
-            /* update the window procedure */
-            OldValue = (LONG)Window->WndProc;
-            Window->WndProc = (WNDPROC)NewValue;
-            if (Window->CallProc != NULL)
-            {
-                Window->CallProc->WndProc = (WNDPROC)NewValue;
-                Window->CallProc->Unicode = !Ansi;
-            }
-            Window->Unicode = !Ansi;
+            OldValue = (LONG)IntSetWindowProc(Window,
+                                              (WNDPROC)NewValue,
+                                              Ansi);
             break;
          }