When logins are required, The CTRL-ALT-DELETE is mandatory since it's the only 'guaranteed' way that you are indeed talking to the Windows login box and not some other program masquarading as it in an attempt to hijack your password. Supposedly the kernel prevents other programs from hooking into that key combo.
Yes, I always thought that. But if you've got UltraVNC running, it has a Ctrl-Alt-Delete button, which sends that keycode to the target computer. And it works. Which I thought was odd.