LinuxMCE is a free, open source add-on to Ubuntu including a 10' UI, complete whole-house media solution with pvr + distributed media, and the most advanced smarthome solution available. It is stable, easy to use, and requires no knowledge of Linux and only basic computer skills.
See it in a demo video:
They have been dugg so the sites a bit iffy as are some of the components required by this installer (it can download what is needed from their servers but they've taken a hammering).
They are working on setting up torrents for the ISO though so keep your eyes peeled on the site. There's a demo video on Google video which they will link to from their site which shows you what it can do.