Xbox Resource Center has a slick guide to getting MythTV, the open source PVR software, running on your big black xbox. Of course, the Xbox has neither a TV tuner nor the horsepower to encode video at a reasonable rate, so you're actually going to set it up as a front-end to a copy of MythTV running on your tuner-equipped PC. It's fairly straightforward, but will get you up to your elbows in Linux terminals and assumes you already have a mod chip in your Xbox.