Not that this is a guaranteed fix--but try starting up the computer with the Windows setup disc. This would startup the new installation, but instead of installing there will be an option for repair where it goes through and makes sure that all the correct files are there and replaces them if they aren't.
Also, if you have the computer set to save restore points then try entering Repair during the system startup (without the Windows disc). It should bring up some options and I believe the Restore Point menu stuff is there.
Also--Make sure you save any important stuff to a disc before trying anything--just in case.
Also--if you have a Windows Vista installation disc, you can reinstall Windows Vista on the same drive and it'll just place all your old Windows files and folders in a new folder on the drive called Windows.old so you would be able to reinstall windows without deleting anything (make sure you have enough hard drive space though, an additional 15GB for Vista).
I'm not sure if XP does that with old windows installations, haven't tried that before.