Try the
CCCP. Combined Community Codec Pack. Before you install this, uninstall any DivX, Matroska, OGG, AC3 and other codecs and players other than Quicktime and Windows Media Player. (Unless you've DivX Pro which you use for encoding video, then uncheck DivX when installing CCCP.)
CCCP will set you up for playing Matroska (.MKV) Ogg Vorbis (.OGG and .OGM) AC3 audio files and many others in one neat package that clutters your system much less than installing all of what it does seperately. CCCP also handles soft subtitles in Sub-Stataion Alpha (.SSA) Advanced Sub-Station (.ASS) and all the other common ones used in AVI MKV and OGM video files.
If you just want to know what codecs the video and audio streams are in a video, there's
AVI Codec and
GSpot Codec Information Appliance.