Hi
You will need to disable/remove in_avi.dll (Winamp\Plugins folder) to make sure Winamp uses in_dshow.dll for AVI instead.
After removing in_avi.dll, reopen Winamp and make sure AVI is in the Extension List in: Prefs > Plugins > Input > in_dshow > config.
If the problem persists, we will need more info (List of Plugins, dxdiag log, etc)
http://forums.winamp.com/showthread.php?t=161361#video