Gender: Male Location: Chaos.
There can only be CHAOS!
There is no ".avi" codec. AVI=Audio Video Interleaved, or something of the nature. There are many codecs, (divx, xvid, mpeg4, h.264) that will take the .avi extension. DIVX may solve the problem, but it's not the end-all be-all avi codec by any stretch of the imagination. You're much safer finding a K-Lite Codec Pack, just google that and you'll find it; has every codec you'll ever need, up to 2005, at least.
How do I change the main media players of the computer? I'd rather be watching my movies on the normally appointed DVD player than this dinky program I downloaded...
The program should have an option in the preferences menu.
If it doesn't then hold down the shift key while right clicking the file. Then select 'Open With...' and select the program, making sure you check the checkbox for always using that program.