Hello plav and welcome to Seven Forums.
I think the problem is you're not using an elevated command prompt.
In the Start Menu search box area type cmd
, then right click
(at top), and click on Run as administrator
. This should open up a black screen with white letters and you should see this: C:\Windows\system32>_
The cursor will be flashing so just type in sfc / scannow
and click Enter. (Note the space between sfc and /scannow.) The scan could take quite a while. See this tutorial starting at Step 6. Elevated Command Prompt