Additional Domain Controllers

I’m trying to set up a Windows 2003 server (standard ed.) as an additional domain controller. The primary domain controller is running windows NT 4 with the latest service pack.
When I try to add active directory in my win2k3 server it asks me the domain. when I click next it tells me that there is a DNS error and it cannot find the domain. I can log into the domain just fine but it wont find it when I’m trying to add the additional domain (or backup domain).
Has anyone done this before? I’m pretty new at this and need lots of help.
Thanks in advance