Never EVER use a codec pack.
Use VLC/mplayer/media player classic for all your video needs, they do not need any codec pack but play any video. Same for Winamp for audio.
Codec packs can cause all kinds of trouble, stay far away from them.
Contradicting post... All those players install their core codecs if you lack them. MPC even installs FFD and supports it. KLite installs FFD with MPC. Pick one, you get codecs either way.
I'm using KLite and have had zero issues with it. The CCC codec pack is one that should be avoided and actually causes issues, KLite updates in a much more reliable time frame.