JSI Tip 1079. Windows NT Workstation users receives "The Domain Is Not Available".

If you are sure that your UserName and password are correct, it may be that the machine account for your workstation is missing.

Use Server Manager on the PDC to remove the Windows NT Workstation from the domain. Wait 15 minutes for replication to occur and re-add the Windows NT Workstation to the domain. Wait an additional 15 minutes before logging on.