I suggest upgrading to Windows 2000, you won’t lose anything!
I know you really don’t want to hear that, but Win2k is great, and it’s the only advice I can give you because I myself got here looking for how to open this filetype ^^
You should use a player called VLC PLayer. You can download it at this website http://www.afterdawn.com/software/video_software/video_players/vlc.cfm and if you need help after that read this
there are 2 much better programs that can be used and you don’t have to change what windows you are using.
The first is Wondershare, any of their video programs will run almost any video file and or convert it to what ever player you want to watch it on, they do have free trials but I wouldnt use it cuz it only converts a bit of whatever file.
the second option is there is a codec you can get for itunes.
so there you go. dont listen to idiots just research it( type .mkv into your search bar)
Many video players can open MKV file such as VLC media player, but if you wanna get MKV generally accepted by all video software, try Pavtube Video Converter to convert MKV to MP4 or other video format, the whole process is insanely simple.