K-Lite Mega Codec Pack includes almost all the audio and video codecs. After installing, the WIndows Media Player will be able to make use of the codecs. The Mega version contains VFW/ACM codecs for video encoding/editing (in supported applications). The Media Player Classic version is included with this program itself.
This program should only be used to install only those codecs you need. Installing all the codecs might slow down the system, cause poor quality video play, etc., since it will automatically substitute the codecs available in Windows with the free, unreliable codecs.