Do you mean Codecs? I use to have this problem, not being able to play a movie file or not be able to listen to the audio from a movie file.
http://www.free-codecs.com/
On the left panel, named "Featured", click on "Codec Packs"
There are different types you can choose from but most of them will have what you need.
I usually go with the "K-Lite Codec Pack"
(There is also xvid.org, look it up to read more about it)