Adding to a domain

I've just installed windows nt server with sp6 onto a Pc. I'm trying to add the server to my domain. The server will be on a network with 3 addtional servers. It will act like a pdc along with the others.

When i go into the section to add the server to the domain (network/properties/identifaction changes) type the domain name, its gives the following message:

<<the domain name entered is already is already in use by other computers.>>

Why can't I add the new server to my existing network. How can I add it

 

case closed. i figured it out..