Nim,
To be quite honest - I wouldn't expect an application to be able to hijack an operating system in such a way.
I can't see a 'child' process (your app within the op sys) being able to override the op sys, because it sit's within the op sys scope.
If this IS required, then you can try <keypress>, <on key>, <key up>, <key down> events to trap specific key presses (CTRL, ALT & DEL).
You would have to stop ALT (due to ALT + TAB switching applications within Windows).
Can't say I've tried it, but can say I don't believe that Microsoft is so stupid as to allow it!
Good luck.
ATB
Darrylle
Never argue with an idiot, he'll bring you down to his level - then beat you with experience.