what's the best codec for .mkv playback?

hi,

this is my first time to post in the software forum and i'm hoping someone will be able to help me with my problem. i've been having some issues in playing matroska (.mkv) files as the video lags behind the audio. the audio is fine but the video plays like it's in slow motion. i've been using klite mega codec pack and media player classic for a few years now but i've only experienced this problem lately. this problem comes out most notably when i play 720p mkv files. when i play lower resolution mkv files, they play just fine. i believe my system's good enough to play these kinds of files so i don't know what's wrong. i've heard about coreavc but i'm not sure if it's already in the klite mega codec pack.

here's my system specs:

windows xp pro sp2
pentium dual core 3.0ghz processor
2gb ram

i'd really appreciate the help. thanks!