Are you sure you havent recently installed or updated a driver?
Often if an incompatible or bad version of a driver is installed, it can cause the BSOD (Blue screen of Death) which isn't always displayed, as the machine tries to reboot to get past the problem.
I'm assuming its XP or Vista?
Try booting into safe mode, if you can do that, then its almost certainly a bad driver, and a system restore may be your answer.