I'm not 100% sure about this, but I think you'll have to have a domain controller in order to have a domain (something to keep track of usernames, workstations, active directory and stuff). Like I said I'm not 100% sure so you may want to get a 2nd opinion.
i'm not serious enough to buy winserver or anything like that...its more of a curisity thing...i'm trying to build a pc now which i'll just use as a local server to sort of experiment with and teach myself more about networking...thats when the damn thing will show something on the monitor