Win 2k on NT4 domain (Active directory)

I have an NT4 domain and I have just added a Win 2000 server to this domain (no problem), I want to set up VPN through the Win2000 server but need active directory to make it work.

My question is how do I setup the active directory on the Win 2000 server when the NT4 server is NOT an active directory server? (thats the message I get when I try to add the Win2000 server to the NT4 server as a child domain).

Any help greatly appreciated.

Simon