If this is a brand new computer, the first thing I would do is call who ever I purchased it from :) It should certainly be covered by warranty. Sounds like a fan or power supply issue. The newer processors require proper cooling and adequate power supplies. If there's a mis-match.....you have problems. XP Pro is a wonder....it really is. The rebooting as you described is what XP does instead of the blue screen thing. I've only experienced it twice in three years.....defraged and the problem went away. On a brand new 'puter however, defragging should not be an issue.