W2K Pro clients cann’t see domain.

We are running a Windows NT Server 4.0 domain and problems occur when Windows 2000 Pro clients try to join the domain. Windows 9x clients have no problem with joining the domain. The environment is a routed TCP/IP network. When certain, not all, W2KPro clients try to add to the domain they get an error message that says” The domain MSU could not be found. If netbuei is added to the protocol list then the clients is allowed to join the domain.