.mkv is a video container, just as .avi is a container. .xvid and h.263 are your video codec. Your player is missing the necessary codec to play the container.
My suggestion is to uninstall all your current media players and codecs.
Download
Combined Community Codec Pack
or
VLC media player - Overview
Along with CCCP, download
Real Alternative and
Quicktime Alternative
Almost all codecs are supported. CCCP is designed specifically for anime.