You may have fixed this problem by now, but here's my two-cents--just in case. As it was said, it could be hardware; or it could be any of the important files that allows Windows to start, are corrupted. I've seen this happen before--even after a clean install. I suggest trying to re-install and if it still doesn't work, re-work your install cd(s) and try once more. After that, if you're still having this problem, look into testing the hardware. Also, even if this works, your hardware could be causing corruptions in your data. Check you ram for errors and test the fitness of your hard drive. You can download apps that do this for you.
