Windows – make Ctrl+Alt+Del go directly into the task manager in Windows 7

Ctrl+Alt+Del is handled at the lowest level of the OS and cannot be overridden by user-space programs. This is why Windows requires Ctrl+Alt+Del before logon - this will ensure that the keys following the Ctrl+Alt+Del be handled by Windows and not other software.

Most likely your program that hangs has completely stopped the OS from running (e.g. causing memory/graphic card trouble and so on).

If UAC is disabled on the target computer then passing a fake Secure Attention Sequence (SAS) keystroke (Ctrl+Alt+Delete) to the system (ie: from an OSK or a remote-control program like VNC) will fail.

If you need lowered UAC interference, then you can turn it down to one notch above Off and that should allow the SAS to be passed.

Similar question is here: Send Ctrl+Alt+Del over VNC with Windows Vista and UAC off?