Download CCCP, its a free build of codex and will allow you to play MKVs in WMP, it also comes bundled complete with hands down the best media player out there the Media Player Classic. Its a safe download, no bloatware, adware or viruses.
http://www.cccp-project.net/
problem solved, you'll be able to watch any video format in WMP just fine once you get that. enjoy!