Under Linux (and probably other x86 'NIXes) ctrl+alt+del either, in the "soft" case, sends a SIGINT to the init process which calls whatever is defined in /etc/inittab. In my case:
# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -r now
or the "hard" case where it just restarts the system.