Due to the slow start of Windows Vista/7, I never really needed to figure out how to repair those installations and just kept on repairing Windows XP installations by booting the XP CD and selecting the repair option. Now that people are really starting to use (and consequently screw up) Windows Vista/7, knowing how to do a repair is actually pretty important. Unfortunately, Microsoft changed the wording on everything. It’s now called an upgrade (even though it’s the exact same version of everything). It preserves settings, programs, files, etc and just repairs the operating system.

http://www.sevenforums.com/tutorials/3413-repair-install.html