I had this same problem once and it turned out to be the display adapter card that was plugged into slot had come loose for some reason, You might check that. Also check your Fans to be sure all are running and it's not a heating problem. If the problem is none of those and you can't boot into safe mode the only other option I know is insert your retail copy of XP and do a repair
installation....then redo all of the updates.
However, there are some really smart knowledgeable people on this forum so you might wait on the repair until one of them drops by with better solution.
It would really help if you listed your computer specs.