Don’t like codec packs? Try this: MKVs for Minimalists on Windows 7
The first step to playing MKV (Matroska) video files in your Media Center is to make sure your system is capable of playing MKV files in the first place. There seem to be innumerable ways to accomplish this and everyone has their own preference. For the sake of this small post I'm going to recommend a codec pack. Codec packs are often shunned as being bloated and containing conflicts within their own packages but I've found that the Shark007 Codec Pack is an unbloated and simple solution that just plain works. Currently Shark007 has a codec package specifically for Windows 7 in beta and the x64 Components are well tested and will work in Windows 7 as well.
For Media Center, users running an x64 version of Windows 7 who wish to use the codec pack will need both the codec pack and the x64 components. Users running an x86 version of Windows 7 will only need the basic codec pack.
Remember, there are many different ways to install mkv support on your Windows 7 machine, Shark007's codec pack is just one solution that works in both x86 and x64. There are many other resources around that can help you set up mkv support without codec packs and even alternative codec pack solutions to be used. If you’re looking for a more light-weight install try this guide: MKVs for Minimalists on Windows 7.
The second step is to get those mkv files displayed in Media Center (and Windows Media Player as well!). This step is even simpler than the first. To get mkv files to display in your Movie library all you need to do is install a simple registry fix.
Download either this x86 reg file or this x64 reg file to your desktop then double-click to run the file. A dialog will appear asking you to confirm adding the file to your registry, click to allow. Then restart your computer and voila! mkv files should now appear in your Windows 7 Media Center Movies Library (provided you setup your DVD Library to point to a directory containing mkv files).