http://cccp-project.net/

although it was designed to play anime, it has pretty much all codecs that are ever needed to play online video content that isnt streaming

it plays xvid, H.264, mkv format, and many others
included in the install is MPC and Zoomplayer (standard)

this pack is made by people who know what they are doing so it installs without problems and uninstalls cleanly

+1

It is officially endorsed by the developers of Matroska and unofficially by On2


Its installation FIXES damage done to systems by other codec packs. Its insurgent, an optional download, when well wielded can rescue a suicidal system.

It is a must have! I carry the latest build on my Flash USB drive to share with others.