Generally, it is NOT recommended to install codec packs. They bung up the system and alter priorities in registry for different format playbacks.
http://www.howtogeek....y_Codec_Packs_Are_Bad
I learned this some years ago when I started using Zoom Player.
Most formats can be displayed simply by installing FFDShow codec. It is a small multi-format codec that will play all XVid, Divx, MP4, and many others.
http://www.free-codec.../download/FFDShow.htm
For DVD playback in any DirectShow player (like WMP), you will need a splitter, MPEG2 decoder and AC3 filter. You can get there also.
For Real and Quicktime playback, just install the Real Alternative codecs, and the Quicktime alternative, one of them with Classic Media Player. Although not totally necessary CMP has it's own codecs, and can play content that will not play in others. VideoLan's VLC player is the same.