SBS 2003 Login loops

I’m running SBS 2003 and all of a sudden I can’t login. I put in the correct username and password and it accepts it and then go back to the login window. I have tried Safe Mode, remote desktop but nothing works. Users can work on the programs and files that are shared and the exchange server works but I can’t get into the system to update and make changes.

Any ideas on what is happening and how to correct it.

Thanks
Howard