We've been able to duplicate this on two machines.
Windows 8.1 Pro
After installing Optional update KB3000850, the machines hang up - they cannot run network scripts, nor run many programs on the system. Some work, but much doesn't.
You can't even restart or shutdown - it just hangs forever.
We noticed that on both systems, the Avast system tray icon never appeared. So we thought to remove Avast.
If you get into MSCONFIG and force the system into safe mode - remove Avast using the Clear program - the system starts working perfectly.
Something about that KB messes up Avast. Please make sure you avoid that KB for now until the Avast team has a chance to check it out.