computer shut down

Lately, when running a particular program, especially the Microsoft Security Essentials, after appx. 6 minutes the screen goes black. I'll move the mouse and my administrator icon appears in the center of the screen, (other icons are missing) under the icon is my name, under that "Locked" and below that "Switch User". I click on the icon the screen opens to MSE and it continues for another 6 minutes.
It is really annoying - my only thought is to do a complete reinstall.

Just to add. I don't have to be running a program for the screen to black out, it happens while I leave it at the desktop setting.

I decided to run SFC, tried again and it appears to be working fine.

I'd also check for infection, extablish a Clean Boot for best performance, and as time allows go over the other Troubleshooting Steps for Windows 7.