that depends on the encoding of the avi file, some can be simple old style avi, or some can be compressed divx format.
you can try the windows media player for them, if they work then your good, if they do not play then you might want to download the divx bundle.
http://www.divx.com/divx/
if you dont like the idea of ads, you can prolly just download the divx codec pack and it should allow your media player to now handle these files, again, only if it is required or you want.