shutdown & reboot

My pc has been shutting down and rebooting every 2hrs. Has caused my admin tools reset. Has shutdown av protect, and reset faults on my za firewall. Have cleaned my fans which really not that dirty. Had sp2 installed when it came out, and has been functioning prop since.Have run all kinds of virus and worm scans and found nothing. Shut down auto startup also. Running amd 1.25mhz w/256mb ram…..So confusing……Help!!!!!!!!!!