The best solution:
1. Go to
Tools -> Options -> Video in MC. Set the Playback Method for
ALL TYPES to Automatic (especially MKV). You usually don't need to change these unless you want MC to act differently from the rest of your system. This should be rare, and only for special circumstances when you know what you're doing.
2. Go to the Add/Remove Programs Control Panel and completely uninstall CoreAVC, Haali, FFDSHOW, and DirectVobSub (or VSFilter if it is listed). If you have any codec packs, remove them too.
3. Reboot. Don't skip this because you are lazy.
4. Download the latest build of CoreAVC if you aren't sure you have it. Also download
the latest CCCP Beta from here.
5. Install them in THIS ORDER...
6. Install CoreAVC. Get it licensed and set up.
7. Install CCCP.
8. On the bottom left of the first page of the CCCP Settings dialog that comes up, make sure
CoreAVC is enabled under the
Alternative H264 Decoders section.
This will probably fix you right up. If not,
follow these instructions for me and paste the results in this thread.