I had this issue on 6 computers at 2 different buildings. I was able to fix the issue by restarting into Safe Mode, then running a chkdsk /fix. After reboot I could log in normally. Then uninstall SAV 9 or 10. That is the culprit.
When I initially saw this issue, I was able to fix it with just the chkdsk /fix, but the issue came back within a day or two. I haven't had the issue over the past week, since removing SAV, and installing an updated Symantec Endpoint Protection.
I saw this page, and dashman's response helped me narrow down the problem, since that was one of the common apps.