Windows 2000 clients on NT4 network

We have a Win NT 4 Server LAN in our organisation and have recently upgraded the clients from win 98 to win 2000. However I often face problem joining the AC domain after a fresh installation of a new PC on the same network. It gives errors like “conflicts with an existing set of credentials” or “your domain interface cannot be found”. Can Any one help? faisal@austroconsult.com