My friend has this problem with this admin account on his Windows 7.
His guest account works.
I found a microsoft help page and tried two of the solutions so far - both failed.
1) If i am logged in as admin "safe-mode" or guest "normal-mode" I can not create a new user account through "control panel - user accounts" - the functionality simply does not open/work when I click it (this was my initial idea - to create a new admin account and move data over to that)
2) If I run microsoft "fix-it" for he issue on the guest account - it runs, but nothing is changed. I can not successfully run in on the admin account in safe mode.
Right now, I am a litle at a loss. I had this problem myself once, but I had a backup admin account that worked which I used to fix the problem.