really depends on the spec of the system.
if you basically want it as a media server. Then i would go for something like Mythbuntu.
its Ubuntu with MythTV built in.
MythTV can be run as a backend, serviving media files, a front end media center, or both.
u you can pretty much stream off it from any OS, and if you plan to have a dedicated media center to play stuff, then you can use a front end that works well.
it also uses considerable less space, and less system resources than something like vista or windows 7