Win2003 Domain with NT Server

Ok I’m sure this won’t be an easy response but here goes….

I have recently inherited a windows network which at best just hangs together. I have 3 servers, 2 with 2003 installed and 1 with NT 4.0. Each server is located in its own domain with its own name although I can get to the shares on each computer when logged into any 1 of the domains. I am now trying to add another 2003 server onto a domain with 2003 already installed but it won’t let me run dcpromo without first running adprep (as I would expect).
Q..? If each server resides in its own domain (but all servers can see each other) what will adprep /forestprep do with the NT server.