depending what games you play linux has been known to cut it quite well in a gaming server position.
personally id use openbsd if its just for office crap (ftp, scp, ssh, http, irc, IMd's etc..). once it starts cranking itll stay cranking for a very very long time, even when you dont bother to update it
(just dont install cvsd)
any server edition of windows will cost you more then the computer is worth i imagine. (then theres the fun of having your server being broken faster then you can download from windows update).