Aside from the Microsoft codecs, here are the ones that I have installed which I consider essential:
Video
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Indeo 5.10
Intervideo Video Encoder/Decoder
MPEG1 Video Decoder
DIVX 5.05
XVID MPEG4
Nero Video Decoder
Indeo Raw
Audio
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Indeo Audio
Fraunhofer MPEG Layer 3
Intervideo Audio Encoder/Decoder
Ligos MPEG Splitter
Nero Digital Audio Decoder
Please understand, these are meant to represent the minimum codec set necessary to supplement the Microsoft codecs. I currently have 59 video codecs and 47 audio codecs installed. However, they do not clash.