If you have positively elimated pests from the probable causes, you might start looking for faulty hardware or software. XP can be set to restart by default as a way to deal with a fatal error. You could have a weak power supply, a hot running processor, bad RAM, etc. You can try reseating components, cables and such and run with your case open with a fan blowing in to rule out a heat problem. If you find heat to be the culprit, I would clean the dust from fans and the processor heat sink. Check the wattage on your PS to be sure it's more than adequate for the job.