For any fix, such as null__'s, to work you still need to be logged in to a working profile. The Admin profile in safe mode is most recommended, but a second user profile with Admin privileges can work, also.
If you are running Microsoft Security Essentials, there has been an issue with a Windows update, but if you have the latest version of MSE then this shouldn't be an issue.
So those are two ways to fix the problem. But if you can't log in anywhere you will need a bootable medium or slave the hard drive to another computer to try to fix it. If no joy there then Louis is right, you just need to give up on the corrupted profile, make a new one and copy your files over to it.