I (well Roger did most of the work) just upgraded my computer. New motherboard, new processor, new SATA hard drive, new RAM...XP didn't like it. We had the same problem you're describing. It wouldn't boot...just kept rebooting over and over. We had to load from the XP installation disc and do a "repair" rather than a full install. It took quite awhile, but once it was done I was good as new. (Well actually better since I added all the new stuff!)