I'm having the same problem described here:
I have a windows server 2003 machine where when I logon it immediately logs me off.
The answer is to log into the server and use the remote registry to clear out a certain registry entry.
As I write this I'm at a clients office and have run into this. How do I log onto the server from one of the networked computers, since it logs me off as administrator immediately after I log in. The office opens in less than an hour. Safe mode and last known good gets the same result.