If you are certain everything is in order, why not just turn off the automatic restart feature when system failure occurs?
Right-click My Computer - choose Properties - then choose Settings under the heading called Startup and Recovery - there you uncheck the the Restart button under System Failure., and the computer should stop that restarting business.
I am running Windows XP Home SP2 on P4 1.8 GHz and 512 RAM. Recently, my PC started rebooting by itself; when it reboots, I get a Device Error/fatal event message. Initially, it said Display Device, so I updated to the latest Firmware for my SIS 650GL card. The problem continues, but instead of Display Device, it just says Device Error. My Device Manager doesn't have any yellow "!" or other warnings.
I have scanned for Viruses using Norton Anti-Virus and MicroTrend Housecall, ran Ad-Aware and Pest Patrol. Nothing was found.