If you want to tinker with AD for free Microsoft have quite a few pre-configured VHDs for Microsoft Virtual Server.
http://technet.microsoft.com/en-us/bb738372I've used these in the past, mainly the Exchange servers for evaluation and testing. With the standalone 2008/2003 servers you can easily create an AD environment just by running the command DCPROMO from RUN. A simple wizard will configure AD on your server.
Steve.