The BlockInput function blocks the user’s input. However this worked in Windows XP and older but not more in Windows Vista. Now the process must run with Administrator privileges or be in the SYSTEM account.

Additionally JwaWinAble.pas reports about this function:

This returns FALSE if the caller doesn’t have permissions to do this esp. if someone else is dorking with input. I.E., if some other thread disabled input, and thread 2 tries to diable/enable it, the call will fail since thread 1 has the cookie.

BTW you cannot block CTRL + ALT + DEL.