Well, if your windows is seriously damaged because of some missing files, the only way is to reinstall it. Reinstalling windows over an existing installation - big no! :S
Even if you went to a repairshop, the only thing these nerds could do is trying to repair the windows installation. But before revealing my precious hard drive to people I don't know, I'd rather do it myself.
Anyway, I once formatted my notebook like 2 times a month (ok, I tested out various OS's), but this doesn't take longer than 2-3 hours (I like to format c:\

). And afterwards, your PC will be so damn fast!
EDIT: Rather the "format c:\ of purity" than ".. of doom".
