MissingRemote published a guide to building a HTPC, you can read it over here.
There are a variety of reasons why you might be interested in building a home theater PC (HTPC); perhaps you are intrigued by the idea of consolidating all your music, movies, and pictures into a central location. Or maybe the cable company upped their rates again and your only means of striking back is to build your own digital video recorder (DVR). Actually it's my personal opinion that most people build HTPCs just because it's fun! It is always a great feeling showing off your system to someone who's never heard of an HTPC and they can't quite figure out how you can have so much stuff available on your TV. This beginners guide is an attempt to introduce a relative new comer to the world of building your own HTPC. Hardware was chosen with a beginner in mind; a balance of cost, performance, flexibility and simplicity were considered when choosing the following components.