Many videos nowadays use a format called MKV, a very versatile container that can store videos, audio, pictures or subtitles in one file. Unfortunately, Windows 7 does not come with many codecs pre-installed. So, you’ll either have to download codecs or a good media player.
Media Players that play MKV files
I clearly prefer the latter one. A player that includes codecs will save you a lot of time and trouble. One of the media players that includes many codecs is the VLC player. The VLC player also includes a codec to play MKV files in Windows 7.
Other players that play MKV files are:
jetAudio, CorePlayer, DivX Player, GOM Player, Media Player Classic, MPlayer, SMPlayer, Totem, xine, ALLPlayer, BS.Player, Chameleo, K-Multimedia Player, ShowTime, SMPlayer, The Core Pocket Media Player and the Zoom Player.
Codec Packs that include MKV support
If you want a codec to play MKV files in Windows 7, you could download the K-Lite Codec Pack. That codec pack also includes a codec to play MKV files. Other codec packs like shark007′s Windows 7 codecs also include support for MKV files.
As you can see there are many players that support MKV files and also many codecs that include support for playing that file format.