If you've just installed some Windows Updates, and that ruddy Restart Now button is really cramping your style, here is a quick command that will stop any messages until you actually do restart the computer.
Go to Start, Run (or press Windows Key + R), then type in:
net stop wuauserv
and hit Enter.
It is possible that the service will restart itself, but if that does happen then just run this command again to stop it.
Updated: Applies to XP, Vista and 7, and probably 8 as well.