Unable to logon to W2K domain

I have done an in-place upgraded on a NT4 PDC with no BDCs in a test lab. The upgrade went well except I cannot logon to the new W2K domain with NT4 workstations (W2K Prof logons OK). In order to make it work, I have to remove the NT4 computer account from the Active directory computer list & recreate a new computer name. As far as I know, W2K DC is also a PDC emulator, why NT4 workstaion cannot logon to that? Can anybody explain to me why?