I don't think you can remove Media Player 7.1 or later, Microsoft kind of integrates it into Windows. But you can still use mplayer2 all the time, that's what I do to avoid v8

As for DivX, I believe it goes over version 4 and later, but version 3 remains. It's not a big problem as codecs are generally very small. Media Player will pick the one that's associated with a certain video format anyway depending on how they were installed.

I prefer ffdshow for all my DivX/XVid/MPEG/WMV1 playback - not only is it a lot more powerful and uses less CPU, but it also has lots of nice on-the-fly filters you can use like deinterlacing, postprocessing (to increase the quality by removing the blockiness of some video), sharpening/blurring, random noise static etc., and statistics too if you want them