The invention of the CTRL-ALT-DEL sequence is most often attributed to David Bradley, one of the engineers who designed the original IBM PC, in 1982. He put the key combination in to make life easier for the engineers, as they frequently needed to reboot their computers. There is another claim for the invention, though: Mike Wise, founder of A-Systems, in 1975.
Why one would be eager to claim this particular title, I'm not sure :)
Reader, former colleague, and friend Doug S. passed along this interesting video clip from 2002. In it, David Bradley pokes a little fun at Bill Gates for having made CTRL-ALT-DEL famous – and Mr. Bill isn't happy about it: