So you reinstalled Windows and it fails at the start? What is the error that you are receiving at the start? Normally, there will either be a black and a white screen stating Windows couldn't start with an error code OR you could see an blue screen with an error code(depending on if you have it not set to automatically reset when that happens). If you still have that Windows 7 CD, then I would try running a repair on it and see if that can resolve it for you. If not boot into the CD's command prompt then type in "bootrec.exe /fixboot" then "bootrec.exe /fixmbr". That will try to repair your master boot record. If that doesn't fix it, then you may be looking at a bad stick of RAM(And yes I say ram because i see this all the time) or maybe a failing hard drive.