Unfortunately my friend let someone else look at his PC and the guy decided to put Norton AV 2003 on with no updates, you can guess what happended next. I clean installed the machine and ran through the update process and put Norton Internet Security 2005 on with all the updates and have been having the problems ever since. I have seen some posts saying this is down to graphics device drivers (even though I have downloaded the latest drivers for his Radeon 7000 from ATI) or even down to SP2.
It could be set to restart when it crashes, possibly due to a conflict somewhere. Do you get a serios error message when it restarts?
Try going to control panel>performance and maintenance>system>advanced>(startup and recovery)settings and untick automatically restart and see what happens ie bluescreen error message. I think you'll need to restart manually if this is the case, then reverse the setting if it comes up with anything.