Right click it & choose Run as Administrator at the bottom of the screen.
At the blinking _ or the > type in sfc /scannow & press ENTER key.
The scan will start.
It takes a while.
If there is a problem you will probably be asked to insert your Windows 8 CD or DVD.
The link below is where I found this information:
There is some good stuff there.
They have a link to how to use a Command Prompt among other stuff.
Well, I hope this helps someone with Windows 8 & its strange new ways of doing things.
Right click the attached thumbnails & choose Open Link in a New Window for best results.
Edited by medab1, 15 November 2012 - 10:33 PM.