Oh, sorry, guess I read that too fast - I thought you said you couldn't reboot.
It might be a virus... I once had a problem where I couldn't log on to any account without my computer rebooting. I had to reinstall XP on it. If this is pretty much your problem, then I suggest you move all important files on to the new partition and format (and reinstall Windows on) it to save yourself a lot of trouble and time.
Happiness is not about being perfect.
It is about seeing beyond the imperfections.