Reboots after 10 minutes Inactivity

Hi,

I have an odd problem with Vista Ultimate SP1. Everything works mint so long as I continue using my computer for any amount of time. If I get up and leave the computer inactive (no mouse movement) for about 10 minutes, the computer blue screens, then reboots. This is repeatable and I cannot find a reason for it. It doesn't matter what application is up.

Here's what I've tried:
disabled screen saver
disabled all power management routines
Ran Ad-Aware (no big issues found)
Ran Spybot (also no big problems found)
Ran full scan using Mcafee (nothing found)
Updated bios firmware to latest version and restored default settings
Uninstalled any junk programs that I had
Ran System Mechanic and performed all diagnostics and cleanups

Any ideas as to what could be causing this? It must have to do with some process which runs after a period of inactivity.

Thanks,
Paul