When you re install an OS, even the same version from the same CD, you are often likely to upgrade drivers that may have otherwise been left at an old version. Perhaps it is not so much Windows itself but a new, less compatible, version of a driver. Video card being the first I would check. You could try rolling back any possible suspects.

That's all I will make uneducated guesses at. Some experts will show up soon.

With great power comes Awesome irresponsibility.