I've never had much luck installing codec packs, there's too much to go wrong. Especially with those packs which try to "help" you, by changing the delicate balances which previously let everything work fine...
My suggestion is to install codecs as and when you need them. Windows Media Player is useful if you want to automatically download and install codecs, but it doesn't find them all; I tend to download individual codecs, and keep them in a folder for when I do a new Windows install.
One thing that might help is reinstalling a DVD player. Some files use odd combinations of codecs including elements from a DVD codec.