I'm a pretty big fan of Windows Media Center. In fact, it's at the heart of my home theater system. Just one wrinkle: I can't play MKV files from within the WMC interface.
In case you're not familiar with it, MKV is a video-file format like Divx and MPEG-4. In fact, it's a format you're likely to encounter if you routinely download videos via BitTorrent.
You'd think that installing the right codec would enable MKV playback in Media Center, but, alas, it doesn't. Fortunately, there is way to enjoy your MKV videos from the comfort of your couch. Here's how. (Note: I tested this on a 32-bit Vista system. Your mileage may vary.)
1. Download and install the Combined Community Codec Pack (CCCP), then reboot your system.
2. Start Notepad, then copy and paste the following lines:
Windows Registry Editor Version 5.00
3. Save the file as mkv.reg.
4. Double-click the file to add the code to your Registry.
5. Reboot your system, then fire up Windows Media Center.
Presto: Your MKV files should appear in your video library (provided you've added the appropriate folder containing your files, of course), and they should play just fine.