Please read this:
The problem is the first fix (work around, actually) MS put out for the "Duqu" virus/Trojan.
If you installed the first one, go to http://support.microsoft.com/kb/2639417
and disable it. Then run your sfc /scannow. Offline. then re enable it. Bet that solves your problem. That's what happened to me.
You might have to roll back part of Security Update MS11-100 but don't do that until you try what I suggested re the duqu fix.
If after reversing MS's first Duqu fix you still can't run sfc /scannow, research MS11-100 to find out if a 'duqu' fix was in there, or if it caused a denial of access to system files. If it did, sfc /scannow won't run. You can probably roll that one specific update back, but only do so when disconnected from the net.
Research that possibilty here: http://technet.microsoft.com/en-us/security/bulletin/ms11-100
However, reinstall it after you finish. It's that important.