What operating system uses the least cpu?

[Annuit Coeptis]

I just installed XP on an old obsolete computer so that I could use it as a media server and torrent downloader and stuff. It has a pretty low quality cpu so I chose XP for the low system requirements and the windows networking capabilities, but now that I think about it I might be able to do the same things with this computer using a linux distro.

Can anyone recommend a linux os that has bare minimum cpu requirements and fancy networking capabilities compatible with windows computers on a windows network?