Assuming you want it on 24x7, and plan to use as a torrent/download box for media, and you have limited users like only yourself, I would suggest 2 options
1. DIY - Build a DIY box, based on Atom so that it consumes low power. You can repurpose your old desktop, but you will face issues with power consumption. Coming to OS, you can use Windows or Linux, and if you want it to run as a headless server, you can use OMV, RockStor etc.
2. Buy a NAS box you can get from Netgear, Seagate, WD etc. But they will be limited in features.
I need more details.