Those temps are fine, But if you have formatted and only run xp with no other software installed that only leaves a hardware failure or failing. one of your componants might be in the process of dying could be your PSU or memory maybe even the motherboard. Have you updated your chipset drivers audio drivers, IDE drivers and graphical drivers lol all drivers you can! update! and maybe update your bios.
If the problem still happens replace your PSU with another and if it happens again replace memory, then motherboard , CPU etc lol until you find the problem! but thats if you have all those spare parts
nothing worse than a clitch you cant solve!. Make sure all your wiring is correct too in the pc.