I'm with granduncle on this one. Codec packs cause more problems than they solve. VLC alone should be able to play MKVs; it natively supports just about every codec I've come across.
If you are using other tools for conversion, then perhaps you may need additional codecs, but again, avoid those "all inclusive" coded packs. Install just what you need. I've reinstalled XP more than once because that was easier than resolving issues with codecs that didn't install or uninstall properly.
Sony KDL-52EX700, Toshiba HD-D3, Motorola DCT-6416, Popcorn Hour A-110, XBOX 360 w/HD-DVD