I recently came across a video file that was .MKV, and had to do some sleuthing to figure out how to watch it. The best solution I came up with was downloading and installing the codec pack from cccp-project.net. Select all of the default options as you go through the installer. After installing on Windows Vista 32-bit I received an odd “String Error” message, but the install worked fine.
To play my file, I still had to use Windows’ “Choose a program to view this file” and it selected “media player classic,” which is included in the CCCP Project installer. After all was said and done, the movie files were playing correctly in the newly installed Media Player classic.