Windows Media Player codecs

[Hangman]

So I have various .avi files that I would like to play, but the default player, Windows Media PLayer can never get the codecs. So I have 2 questions:

1. How do I get these mysterious codec so that the media player will play media?

2. How do I change the default application for .avi files so I can use GOM player or DIVX player without right-click >> Open with etc.

Thanks

 

[Hangman]

Thank guys. It was driving me mental. Now I can get things working the way I like