help build a multimedia pc

I want to build me a very simple PC to put in my living room to hook up to my TV. I already have a 500GB HD in an external case and was thinking I could use that in the PC.

I don't want anything fancy as I might be putting my current PC in the living room anyway. But since I am not sure I'd like to build one anyway. If I don't put the current PC in the living room, the multimedia PC stays. If I do put the current PC in the living room, I can give/sell the multimedia PC to my bestfriend that wants the same thing.

I just want something that will play the movies I have(WMV, DIVX, 3G2, whatever) and maybe do like internet, but definitely NOT necessary.

The cheaper the better, but of course I don't want crap either. I know it's a bit of a oxymoron, but work with me here. Thanx in advance for the help.