Add NT 4 Domain User to Local Adminstrators Group

Hi,I have try to add the NT 4 domain user to local adminstrators group. I'm getting the following message when I'm enter the network security user who have access right to the domain.Login Failure: Uknown user name or bad password"Any one have an idea how to add the NT 4 domain user to Windows Vista Local Administrator Group? It's Windows Vista can work with NT 4 BaseDomain?PS: I managed to add the Windows Vista box to NT Domain.