Had same prob and the only to fix it, after hours of tests and trying different whatnot plus hours on the phone with M$ too, the only thing left to do was to reformat. If you have a dvd burner drive, or a second hard drive, i'd recommend getting Norton Ghost, so u can create an image of yur C:\ drive and store it on a back up drive, it'll turn a 4h installation into a 1h restore, plus, Ghost can fit a lil over 6Gbs on one 4.7Gb dvd disc. And if u keep having to re-install, then yur hdd is busted, and it's most likely the write/read head that's damaged (y you wont find bad sectors on your disk). Hope this helps.