Welcome to the forum, elyk!
AVI is a container file; it can have many different types of audio and video. The formats are called Codecs, this term is short for "compressor-decompressor" which, when compressing, converts audio or video into bits in a file, and, when decompressing, converts them back.
So you would think that you'd have to have XviD and DivX codecs installed on your computer. You could do that, or you could instead install a program called "ffdshow." This is an advanced video decompressor that works with many different formats, including both DivX and XviD.
Here are links for downloading:
ffdshow
DivX 505
Nic's XviD
Koepi's XviD
uManiacs XviD
EDIT: As to audio, you may need codecs as well, if you don't happen to have the right one(s) installed. For example, if AC3 (Dolby Digital) is used for a soundtrack, you would get silence if you didn't have the AC3 codec or an "AC3 Filter" installed.