In the past, I've found that all I need for codecs are:

-codecs downloadable by WMP6: don't need to do anything
-DivX: available at www.divx.com for free (below the three levels of software is a link)
-Nimo codec pack: hard to find on the net now for some reason. I have an earlier version if you like. This also has DivX in it, so try it first.