Match error tested against to the actual error returned by
NtUserGetWindowLong

reactos/lib/user32/windows

diff -u -r1.101 -r1.102
--- window.c	7 Mar 2004 20:38:17 -0000	1.101
+++ window.c	14 Mar 2004 11:27:33 -0000	1.102
@@ -1,4 +1,4 @@

-/* $Id: window.c,v 1.101 2004/03/07 20:38:17 navaraf Exp $

+/* $Id: window.c,v 1.102 2004/03/14 11:27:33 gvg Exp $

  *
  * COPYRIGHT:       See COPYING in the top level directory
  * PROJECT:         ReactOS user32.dll

@@ -1046,7 +1046,7 @@

 IsWindow(HWND hWnd)
 {
   DWORD WndProc = NtUserGetWindowLong(hWnd, GWL_WNDPROC, FALSE);

-  return (0 != WndProc || ERROR_INVALID_HANDLE != GetLastError());

+  return (0 != WndProc || ERROR_INVALID_WINDOW_HANDLE != GetLastError());

 }