actually.. that said, a program called VNC that is used to operate a computer remotely, over a network, has a menu option "Send CTRLALTDEL" which causes the remote machine to display the options box. i think they may have used a low level keyboard hook of some kind though..
did a bit of research and it seems that ctrl+alt+delete is actually a special combination that, though some specific wiring in the keyboard, causes an interrupt to be generated on the cpu with a specific code. windows handles this interrupt and shows the security screen. how you would emulate this interrupt yourself, i do not know. might be good if you told us WHY you want to do this