If you want to give your old PC a new life (as long as it’s not that old!), or if you are building a HTPC (Home Theater PC), Windows 7 Media Center works like a charm. I’ve used both the Windows 7 version, as well as XBMC (there are a few others, but I haven’t used them in years!), and they are both very good. Although, honestly, I do like XBMC for Windows a bit better. Looks amazing, works smoothly, and does everything you want it to.
There are also hundreds of addons and plugins for Media Center to give it more functionality. Legit Reviews shows you how to install Windows 7 with Media Center on your HTPC, step by step. Once you are done there, hit up The Green Button and Hack7MC to get some addons.