i hate codec packs with a passion. though they might've changed in recent incarnations, i've always found massive incompatibilities and enough hiccups to just stick to ffdshow, which itself is a swiss-army knife. it'll decode not only a host of video codecs including all DIVX, XVID and MP4 variants, but will also tackle WMV, H.26x, MPEG and about 20 other formats. the latest nightly builds also include an audio decoder that'll handle nearly every filetype thrown at it - MP3, AC3, OGG, etc, while also providing advanced features such as volume normalization, winamp plugin support and other goodies.
with ffdshow (including its audio decoder), Real Alternative and Quicktime Alternative, you're covered for nearly every format you can think of.