The Wiz is in!
Sounded like a shoddy install, boot from the XP disk instead of the hard drive and re-install Windows, I'm pretty sure there are 2 options on the install screen and select the lesser of the two that allows you to keep your documents and settings as they were.
That would be the software issue.
There are sometimes hardware issues to check though they are unlikely unless you've been messing around inside.
Does the PC restart at the same point every time?
If you need more help, just post or PM me.
09/04/2008
I am the revolution - Sydney, Australia