Constant restarts in Windows XP with SP1

I have installed SP1 into a Windows XP Professional computer. The Service Pack installation went smoothly, but upon the first reboot, the system began to continually reboot on its own. It appears to actually reboot as it is trying to load Windows Messenger, but I can’t be sure that is the problem. I have seen this as an issue during installation of SP1 to Windows XP in some newsgroups, but i could find nothing in Microsofts support site. Does anyone know a cure for it?