Helpful Hexus Hordes have laid out a guide to setting up & running your own home-entertainment server using Linux & MythTV. They use the Ubuntu distribution as the base of their HTPC.

"The reasons to use a PC rather than a set-top box are many. For one, you're in control of the specification - you can add extra tuner cards to record extra channels, or mix satellite, cable and terrestrial in the same machine."