There are several issues that can cause a Windows XP computer to not reboot properly after installing Service Pack 3. Most of them affect relatively specific configurations, and most appear to have relatively simple work-arounds.
Please: do not do anything rash. I have seen a lot of reports of people who reformat and reinstall when they run into this problem, losing all their data in the process. There is often no need to do anything that drastic. First read this post, and see if anything here helps you. If not, call Microsoft's technical support line and see if they can't help you.