Computer Locking up?

without any real diagnostic tools it would be tough to tell with any certainty....it more or less sounds like a power supply issue. That could mean the power supply itself or it could be an issue with the motherboard providing power to the chip....or for that matter it could be a power pin on the chip itself. you're not going to get an absolute definitive answer here....or at least you shouldn't, but an interruption of power flow will usually lock up a processor in a familar way and your syptoms fit the bill. I'm guessing you don't have any diagnostic tools handy but if you got parts laying around like another power supply or can flip out the chip with another to test it out you might get somewhere with that. I wouldn't stress about it too much but if the frequency increases than I would start looking at replacing the power supply....they tend to fail at a gradual rate so you might be getting close to the end with that unit.

When you reinstalled Windows Xp did you put a cd key? If you didn't Windows thinks its a fake copy so it lacks the computer, but if you did then its probably a virus. There you go you added internet which is most likely a virus came into the system.

It could be a virus, spyware or other malicious program. You also should scan your hard drives for errors and defrag them. Have you looked inside the case lately. Sometimes dust gets so thick that the processor overheats. Also check the coolong fan for the same reason.