There was talk of K-lite Codec Pack, the Combined Community Codec Pack and VLC and Media Player Classic HomeCinema
I'll mention what they are and what they aren't.
Codec Packs, both k-kite and CCC are just that: codec packs. They bundle a bunch of codecs and splitters to be installed in the windows video codec system, which is pluggable.
They usually state a list on their downloads page of what's in the pack. You can download and install those separately for the same effect.
I usually just install XVID, RealVideo and MP3 support, and Matroska if I'm in the mood.
VLC and MPC-HC are players.
MPC-HC supports several video containers such as AVI, WMV and others but for playback relies on windows codecs, meaning you'll still need them.
The nice thing about VLC is also the bad thing about it. It has its codecs compiled in the program, so you don't have to install anything. If you update codecs, it has no effect in VLC.
That's why I usually use VLC to play Matroska. Also its a pretty nice DVD player, with no horrible 'deck' or 'remote' control like the other players out there.
Codecs decode streams. Splitters split the streams from it's original file. For some reason the player still has to allow you to open the file to be split ( splat? XD ). WMP doesn't allow you to open several file types such as matroska, so even if you have the splitter and codecs for the file, it doesn't (I think) work.
AVI, RM, MKV, WMV, MP4, QT and other video files are "containers". These files contain the streams, which can be coded in several codec choices, depending on the format. Some formats allow ANY codec, some format allow ANY AMOUNT of streams (MKV!!).
I hope this helps. Please continue video discussion here.