Hello Harry...
AVI can use many codecs... like divx xvid etc.. and wmp11 does not have all
of the codecs nor does it download all of them automatically.
A good tool to determine what kind of video and audio codec your video files
are using is this:
Gspot
http://www.free-codecs.com/download/GSpot.htm
So you will scan your avi file with Gspot and you will read what codecs its
using, and then get those.
Thats how the pros do it.
I would suggest you ditch the WMP11 and use gomplayer that has a huge amount
of codecs preinstalled in its own application
www.gomplayer.com
if you still insist on using the horrid WMP11 you can use a codec pack like
this:
http://www.free-codecs.com/download/K_Lite_Codec_Pack.htm
although I advise you to use gomplayer as I said before.