Ouch, sounds like fun.
If this is a new forest/domain, run the system manager tool and add the Active Directory Services (something like that anyway) role, then run "dcpromo".
Remember to install/configure DNS when it asks.
Tutorial here;
http://www.windowsnetworking.com/articles_tutorials/Running-Windows-Server-2008-R2-Installing-Creating-Lab-Domain-Controller-Part1.html