ExtremeTech compiled a nice article which shows you how to build a Windows Vista home theater PC:
It's been far too long since I last built a home theater PC that I would want to put into the rack with the rest of my home theater system. Last December's build used Windows Media Center 2005 Edition. Now, Media Center comes integrated into Windows Vista Home Premium and Ultimate Edition.
High-definition playback has evolved since then as well. While the format war between Blu-ray and HD DVD continues without any clear winner, the cost to entry has dropped. A greater variety of over-the-air digital tuners for PCs have become more readily available.
However, this time around I'm going to avoid a "one size fits all" approach. Rather, we'll start with a core set of components, then discuss viable alternatives that can hit different budget points..