When I logon to Vista, it logs me off right away.

Hello, something strange has happened overnight. I enter my password at the
logon screen {the correct password}, and it begins to load my settings.
Seconds later, vista logs me right out again, before booting into the desktop.

I've booted up in safe mode, entered a new password, and it still won't
logon. I've removed the need for a user account/password and it seems fine.
However, I can't password protect vista for some reason.

Any idea what would cause this? Thanks!