AVIs can be encoded using different CODECs (Coder/Decoder), and to play
a particular AVI you need the decoder that was used to encode it. There
are many CODECs, and what you need depends on the content of your AVIs.
However, there is one relatively easy solution that I've been repeating
recently: download and install FFDSHOW (latest version from
www.afterdawn.com), and it will handle just about any AVI (and more)
you are likely to encounter.
After you install FFDSHOW, open the FFDSHOW Video Decoder (I'm not sure
of the precise title), go to the CODECs section, and click on each
entry that says DISABLED and choose something other than DISABLED (the
first entry is fine). Or, you can just start off by making sure that
entries that have DivX, Xvid, MPEG4 in their description are not
DISABLEd. I can't be more specific because I don't know which AVIs you
have. Normally, there is no harm in ENABLing more that you need in
FFDSHOW. After you familiarize yourself with it, you can enable/disable
as you see fit, though again, you are not likely to cause any harm by
just enabling a whole bunch.
After you do the above, FFDSHOW will decode any AVI, MPEG, or whatever,
for which it has been enabled. It is a very simple solution to all the
CODECs overwhelm us.