i've got an old Athlon XP1700+ that runs Ubuntu as a living room media server/web kiosk. i really did it for shits since i wiped XP off the drive to put it on another machine and had Ubuntu running so well on it. It definitely consumes more power than it needs to, but 98% of the time it's just acting as a remote file server.
Before that i had a Pentium II 400MHz laptop performing the same function. :lol
Can't really tell you how i did it, but there's a number of Ubuntu networking guides out there. i'm sure Windows Home Server makes it painless, as file sharing between XP, Vista, and Ubuntu has been a pain, but i really wanted a *nix box in the house, and i'm not putting out for a Mac just yet.