The only time I seem to have ever saw repeated and unexpected system restarts was on the last build where something simply overheated once too often. Without a trace of a virus or other malware a damaged or missing even cross linked set of drivers causing a clash would see various problems. XP has it's own "crash control" method of restarting the system if a crashing program is about to totally lockup the system. But I find myself skeptical on just how effectuve that is to start with?!