There are a few advantages that a home-built Home Theater PCs (HTPCs) give you over TiVos or cable provider DVRs. Among them are the lack of subscription fees, the ability to play back DivX and other downloaded programs, and the fact that you can use it as a computer when you're not watching TV on it. This is why, despite CableCARD HTPCs shipping soon, people are still opting to build their own—albeit only broadcast HD—DVRs.
ExtremeMhz has a thorough guide on how to build your own HTPC. They explore several options like Media Center and Media Portal, but are light on Linux options. Despite this, it's still a good start as a primer to building your own HTPC. Warning: building your own DVR requires hours of free time, which can be much better spent with your loved ones, outdoors, or pretty much in any other way. We speak from experience.
Extreme Guide: Build the Perfect HTPC [ExtremeMhz]