If one wants to get the the message string (output) given the message identifier (input) returned by RegisterWindowMessage - i.e. a kind of inverse of RegisterWindowMessage - it is possible to use GetClipboardFormatName :
http://msdn2.microsoft.com/en-us/library/ms649040.aspx
In fact, doing a DUMPBIN /EXPORTS on user32.dll, it seems that RegisterWindowMessageA/W have the same RVA of RegisterClipboardFormatA/W.
So, basically, it seems that RegisterWindowMessage is just an "alias" for RegisterClipboardFormat...
For more details, read the thread on microsoft.public.vc.mfc newsgroup, with subject "Registered Window Message String", started on 2007, September 7th:
http://groups.google.it/group/microsoft.public.vc.mfc/browse_thread/thread/f83f7c12c80e4ada/460bc4c43a844a37
Giovanni
MSDN says: "The RegisterWindowMessage function defines a new window message
that is guaranteed to be unique throughout the system."
On vista 64 RegisterWindowMessage(theSameString) function will return different values for programs run under localsystem and programs run under any other user.
::PostMessage(HWND_BROADCAST,registeredMessage) returns TRUE, but GetLastError gives access denied (error 5) when message is posted from an elevated process to a simple one.
Iohanson