It sounds to me like a
CODEC problem. You need xtra ones that will recognise the videos. There's numerous types of AVI. AVI is just the outer shell, what's inside could be a great many variants.
A list of
CODECs sources can be found here: 1.
Free-Codecs.com, 2.
Nimo’s codec pack, 3.
CoDecs & codec identifiers, 4.
K-Litecode Codec Pack
Also consider getting a backup player with built-in codecs such as VLC:
http://www.videolan.org/vlc/ (choose nearest mirror site)
WMP sucks! as a player it really does. lol