Win2K domain controller

I recently upgraded my NT 4.0 BDC to WIN2K server but elected not ot load AD, so it is just a member server. I still have a NT4.0 PDC. I now want to make the WIN2K server a BDC, but when I go through the process of loading AD (as an additional controller) it wants a Domian name, our domain name is NTSERVER, but it says it can’t find it although it communicates fine on the network and can find and be found by any other computer on the network. Do I need to select the “domain controller for a newdomain” option during AD installation even though I want to continue using the current PDC as a PDC? Is it possible to use an NT 4.0 PDC with AD?