I really want one of these for my Windows Media Center....
http://www.trustedreviews.com/article.aspx?art=874
The HushPC is completely fan-less and uses passive cooling techniques to cut down on noise pollution in the room (more than I can say for the current machine sitting in my living room). It's a little on the expensive side for £1,791.38, but is well made and looks so darn cute.
Buy your HushPC at http://www.hush-technologies.net/