I have Windows Media Player, and Whenever I try to play an Avi. file, my computer says that it doesn't have the codec to play. I have looked online and google, but I haven't got a clearcut answer just by searching. I don't want to download the wrong thing either. Any help is appreciated.

Doesn't play ANY .AVI file? Try to play other AVI video files if they work. If not, then that means you don't have the necessary codec to play the video.

AVI is just a file extension, the codec could be an XVID, DIVX, etc...

Try vlc player... It plays anything and everything... Just google search for it.

Or for codecs...
http://www.free-codecs.com/K_Lite_Codec_Pack_download.htm

Vlc is the only thing I use to play movies.

Win amp is also good for most stuff

VLC Player is the best. Also you can try the codecs from the link I mentioned above to play em in Media Player