I'd stay away from codec packs, any fansub group that puts one out is just supplying the versions that
they use for their releases. Installing the major codecs individually and using a superior media player is a better idea than trusting some 17 year old kid on the internet that knows how to zip up some files.
now these are just my recommendations as they're what I use but I've had no problems or slowdown on anything at all.
Media Player Classic, opensource media player with the look and comparable system usage of WMP 6.4; better functionality to run container files like .ogm (ogg vorbis) and .mkv (matroska).
DivX 5.2.1, a given to get even though not as many use it anymore.
XviD 1.03 - Koepi's binary, more commonly used opensource version of DivX.
ffdshow 2005-02-16, a multiuse codec that can do a number of other formats.
Matroska Pack lite, bare essentials needed to play anything matroska, use MPC to open .mkvs