I had the same happen to me...however I have a slightly older Celeron 800 that I built 3 yrs ago. Windows 98 always ran smooth. When I was pre-evaluated for XP, my only questionable components were my HP printer(common) and my AOpen56k modem(generic) which I was upgrading to high speed anyway. I did upgrade to 256 ram(recommended).The only way to stop the repetitive reboot, was to unplug. What I did was insert my Win98 startup disk on reboot,format, and return XP for full refund(must be within 30 days...MSN HELP[Tech Service]will give you the number). I'm no techie,but if any of your components aren't Microsoft signature,GOOD LUCK. My guess on my situation(and yours)would be motherboard.