IMO:
MPEG1 Built-in
MPEG2 Built-in (FFDSHOW in case it's not DVD you're watching) (This assumes you have version of Vista with MPEG2 codec)
MPEG4 FFDSHOW (Latest ICL10 build by CLSID from FFDSHOW Tryouts)
I've gone ahead and installed ffdshow and Haali. I setup ffdshow to use MPEG2 also as Vista Business seems to be on the no-list for MPEG2. Most things seem to work fine, although some video files still play wrong, but it might be to those being broken.