Oh Madonna! Have you seen the intro?

[[wcip]Angel]

Download G-spot (no, it's not what you think, you filthy bastards! :P). It's a programme which tells you what codecs you're missing for any particular file you want to play. You open the avi-file in Gspot, and it tells you whether or not you have the necessary audio and/or video-codecs installed. The programme also gives you the name of any codec you might be missing, so googling it, won't be much of a hazzle.

(Or you could go for Deebye's defiler-pack.)