Xp Pro wont start or shut down

XP PRO wont start or shut down.

In start up it gets as far as the load progress bar. That zips across the screen then goes blank Just before it gets to the windows logo. I have to switch off manually. When i do i get a blue screen that says hardware failure, contact your hardware manufacturer.

The question which hardware is causing this?

At shutdown it saves settings, logs off then when the windows is shutting down message appears it just freezes and i have to switch it off at the switch at the rear of the PC.

Any ideas?

P3 800
390mb ram
onboard sound
agp video card