another option is to
1. goto the run dialog, and enter "mplayer2"
2. open a file that is displaying green
//heres where i forget the exacts
3. goto properties, click advanced - and there is a "decoders" or "filters" tab
4. select ffdshow, and look at its properties. it is probably selected to display xvid encodes. this should be deselected.
i usually download codec packs on new installs, and then remember about the green video thing, and have to do the above. i get bored looking for all the individual codecs