Probiere mal, das ganze ohne Debugger zu machen... ich habe mal gelesen, dass Debugger F12 für sich beanspruchen.
Edit:
Zitat von
http://www.swissdelphicenter.ch/de/showcode.php?id=147:
Windows NT4 and Windows 2000/XP: The F12 key is reserved for use by the
debugger at all times, so it should not be registered as a hot key. Even
when you are not debugging an application, F12 is reserved in case a
kernel-mode debugger or a just-in-time debugger is resident.