can’t join NT domain from win2K professi

Hello everybody,
I want to join NT 4 server domain from my win2K professional(I upgraded from win 98, but not be able to join the NT domain it show me the following error message:(I have an administrator equivalent acct.)

“your computer could not ne joined to the domain because the folllong error has occured: The credential supplied conflict with an existing set of credential”
Anyone have any idea ?
Appreciate for any opinion.
Benjy