Unable to Log On to windows. Need help. Dell E521.

Computer starts as normal and boots normally.  When I get to the welcome screen where I choose a user name to Log On, I click on my user name and it asks for a password.   I never set a password on it.  Anyway, after trying any passwords I get an error message that reads:

Logon Message

The system cannot log you on due to the following error:

The specified domain either does not exist or could not be contacted.

Please try again or consult your system administrator.

I tried to log on in safe mode but I get the same error.

I have a Dell E521, Windows XP SP2,  Media Center Edition.

Thanks.