The logoff-message are part of the msgina.dll (windows/system32) and some graphics in the BMP-Area of the shell32.dll. Don`t edit the files when you`re a newbie with systemfile-customzing ...
You can download customized msgina.dlls, search google, ask at Neowinn or at Xpize
Here is one I found ...
If you find some more, feel free to let us know - just to be sure ...
