I have a similar set of symptoms; however, in my case, there is only one user account on the machine. Booting into safe mode, does not help, as both Administrator and the User account exhibit the same behavior: clicking to log on immediately logs the user off and drops back to the logon screen.
If other users can log on, the above post would be the way to proceed; however, if all users are blocked from logging on, you may want to check the following thread in these forums:
http://www.softwaretipsandtricks.com...9814#post69814
We might be able to consolidate and surround the problem.