If you dont really have many codecs installed, get rid of the ones u do, and go for CCCP =)
MKV is not a codec, it is a file wrapper (that often uses h264 for the file codec...).
CCCP is the easiest choice (then use media player classic or something once thats installed).
VLC will work, but generally does a horrid job of the subtitles...
Probably the best option I can suggest... is MPUI.. which is kind of like VLC... but better subtitle support (not entirely sure of its legality... but whatever...)
Before trying the others, I'd suggest trying it:
http://www.3wanime.org/MPUI.zipIt will be uploaded about an hour after I post this message (my connection is slow

), and I will most likely be suggesting for this to be used as the WWWA stand alone player of choice.
Just run MPUI.exe once u've unzipped it! =)
Hope it helps!