No offense at all to you, but if you need to ask this question, then most of the time you dont need a domain. Now if you are starting a course of some sort and just want practice, then ignore my comment lol. But I remember that before I learned how to do it right, I screwed up my network REALLY badly. Anyway, you need Windows 2000 Server or Advanced Server, or Windows 2003 server. One more thing though, if this is just for a home network, it really isn't needed. I played around with domains for a while on my home network (my friend's company shut down and he sold me his win2k server for cheap!), and I concluded that I just don't have enough data/users/computers/etc to warrent setting up the permissions and everything that goes with domains. So I just run a linux file server!