Under NT,
Ctrl-Alt-Delete is used to get the attention of the core system; no
userland app can listen for or issue a
Ctrl-Alt-Delete. Unless
NT has
bluescreened, it promptly pops up a helpful window that can start
task manager, change your password, lock the computer, etc. Compare with 9X which may, if it's feeling like it, present a list of non-hidden
processes that it may deign to attempt to close.
Why does NT require you to
Ctrl-Alt-Delete before doing anything security related? Because only the system responds to a
Ctrl-Alt-Delete, you will definitely get the real change password screen, the real lock workstation screen.
Trojan Horses won't work, as the real
NT dialogue will
supplant them. If you've pressed
Ctrl-Alt-Delete, you're definitely typing your password into the system, and not a third-party program.
Under
linux, incidentally, if
/etc/shutdown.allow is present
Ctrl-Alt-Delete won't cause a shutdown unless a user specified in it is logged in. Stop random people walking up to your server and rebooting it!