safetypee,
The link ORAC posted is an excellent resource for troubleshooting.
I just wanted you to run a Virus Scan with:
Trend Micro's HouseCall
Just to make sure it is not a virus preventing you from shutting down. (It is one of the payloads of certain viruses.)
Take Care,
Richard
P.S. If you would give us the specs on your hardware, and when was the last time WinXP was installed, (was it a fresh install or an upgrade), we might be able to help pinpoint the problem.