You did not specify what type of pc you have nor your operating system. I run WIN7 with a athlon II X4 cpu. I use windows media center to play ALL movie formats. Granted you will need to download some codecs and in some cases (playing flacs) you may need to make registery changes. The two websites below will help you. I built a full blown out pc just for my media room, these are known as HTPC's. They are gaining popularity and you will likey see alot of them on the market by christmas. Google TV, Logitech Revue, WesternDigital's WD TV Live and WD TV Live Hub are just a few, but for real performance and versatility you need to build your own HTPC. Mine Rocks!! Literally.
My system specs are similar to this:
http://www.avsforum.com/avs-vb/showthread.php?p=19553212#ATX%20System:%20Mid-Range%20System:%20AMD