Yep, do what Aragorn suugests... you don't want the initial 'Repair' option, but on the next step, Windows will search for an previous installations of Windows and ask if you want to re-install over the top - this is what you want.
I've found in the past that it doesn't wipe out your Documents & Settings folders, but back 'em up just in case. From what I remember, the FaST thingy just copies stuff like My Documents, Favourites, cookies, desktop settings, etc...
What you could consider for future is to have Windows installed in its own partition and re-direct the Documents & Settings folder to another partition, and then whenever you need to re-install Windows, you know that it's only the Windows files that are gonna get wiped out. It's not perfect as anything installed in the non-Windows partition still won't work as the registry settings won't be there any more, but at least you can see what programs you had installed, as they'll still be present on the non-Windows partition, so you can manually re-install them.
Just for info... this is yet another case where Linux works better by design than Windows. As you don't have registry settings, you can easily have your OS stuff in one partition and your apps/games/other software installed elsewhere safe in the knowledge that you can mess up your OS as much as you like and everything will work straight away if you simply re-install.
...er, sorry I kinda got carried away there