The issue is mostly explained in the title, however here's some more information:
This has happened several times, and I have never personally watched it happen. I've only seen it show up in the Event Viewer when I return to my PC being at the login screen.
This PC runs 24/7 for the most part, and the power supply is an 850w ABS from 2010.
Here are my temps and volts currently as reported by HWmonitor:
I am running Zune, iTunes, Chrome, Firefox, Visual Studio 2012 and Steam, so the CPU is under some load, 5-20%, but nothing significant.