Infinite reboot after SP1

I recently installed Windows XP SP1 and the computer is now in a infinite reboot loop. I moved the hard drive to a computer of the same model and it worked fine. I also tried moving the hard drive from that computer to the one that was rebooting, and it worked fine. Now, I have the computer on Windows XP without SP1 and it is working fine again. Anytime I put the service pack on it, it stops working. Has anyone experience anything similar or can anyone help me?