Problem With Accounts And Passwords

I started having a problem access my server with user accounts.

When I try to access my WHS from my PC when logged in using my WHS accound I get a message asking me to log in to the domain, I enter my account name and password and it says it wrong.

(if I use an account 'admin' and no password it lets me in, strange)

I can log into my server through the console or remote desktop no problem with my administrator account, and can verify that my user account name and password are valid.
Seems to me there is a problem with the domain setup by WHS.

What steps can I take to trouble shoot and repair.

My server is one I built and have been running with no problems for years.

Thanks -