Hats off to Krunker for this encyclopedic post about how to convert a Mac mini into a Windows Vista operated media center, combining the options of a Windows based machine with the form factor of an Apple designed product...
At the end of the day, the Mac mini makes a solid and capable Media Center box. It's even better when you use the system in conjunction with Media Center Extender systems since the Extenders can deal with the actual video processing and rendering. There are of course some limitations with the Mac mini - mostly in the areas of storage and expandability. Storage expansion can be solved via external hard drives or via a network attached storage device such as Windows Home Server (and I'll delve into that in the near future). Additional capabilities can be added to the Mac mini via USB connections although it does destroy the simplistic look of the overall system a bit.
Now if he could only work on the overall presentation of his final setup...it's a little to hodge-podge, wires everywhere for us neat freak Apple aesthetic sticklers.
Plenty of photos and instructions at Krunker.com.