I will try switching out the powersupply. All 4 of the computers have the exact same re-boot problem, so I've been "experimenting" on just one machine in hopes that if I find the problem with this one machine it will be the exact same problem as the other 3 machines. I also failed to mention that I had switched out the RAM on this one machine, as well as I had done a full deletion of the partion, and full re-installation of XP. (sorry, I've been working on this problem for about a week off and on, so I didn't remember everything I had done)
Thanks very much for your feedback!! I will try the powersupply.