Classic Start Menu Pro
Classic Start Menu Pro replaces the Windows 7 Start menu with, well, you can probably guess. Just install and run the utility, then click Start for a blast from the XP past. Classic Start Menu Pro also leverages Windows' Aero features, so you get some of the best of both worlds.
Note: There is a free version of this program called CSMenu, which lacks the Aero interface and some usability improvements.