This has happened to me before, and might be because of a known bug that occurs in some Windows XP OEM systems.

See this article for some information about it. Specifically, check your system event log for the two entries noted on that page (in red). If those are present, this is probably the issue.

Is your Windows XP install from a manufacturer or a manufacturer's rescue\reimage disc?
Or did you install XP yourself?

Unfortunately there isn't a lot you can do to resolve this, if it is in fact the above issue. You may have some luck with the sysprep.exe program (should be on your Windows CD, if you have one), but that is a program really meant for OEMs and you could mess up the computer if you do something wrong.