For divx go to http://www.divx.com/ and I recommend downloading the free version so you dont have to worry about the spy/adware.

For xvid which is one of the main reasons why people cant see videos after theyve downloaded them, at least this is what ive noticed from my experience with people telling me my movies dont work. http://www.divx-digest.com/software/xvid.html this will give you some information and I believe it has a link to the installer. You might actually have to search the web for an installer, but it is a great codec.