Boot in safe mode, and see if it shuts down. To enter safe mode just hit F8 before the "Loading Windows XP" screen shows up. (What I usually do is just hit F8 about a million times till I get to the screen.) Once, you've hit F8, you'll be directed to the Boot Screen, and from the list, choose Safe Mode.
Once in safe mode see if it shuts down properly. If it doesn't then it sounds like a hardware issue.
If it does shut down properly, then try doing a virus and spyware scan in safe mode. Hopefully that'll fix the problem.
<br><br><font color=\"black\"><b>There's No Place Like </b></font><font color=\"red\"><b>127.0.0.1</b></font><br>