There have been a bunch of problems for Windows XP users after upgrading to Service Pack 3. Many of these issues seem to be related to AMD processors and people have had issues in the past when upgrading to Service Pack 2. There are a couple steps to take to isolate the issue and then resolve the problem.
First while the computer is rebooting continue to tap F8 so that you can choose to stop on error instead of rebooting. Once you have reached the Windows Advanced Options Menu choose the following option so you can obtain information about the error causing the reboot: Disable Automatic Restart on System Failure.