AIV? No such format.
I'm sure you meant to say AVI. Right?
So, they are not MKV files after all.
Anyway, the procedure is still the same as J_Darnley indicated above...
Open Winamp preferences and make sure that AVI is a listed extension in the DirectShow Decoder...
Ctrl+P for Winamp Preferences > Plugins > Input
In the right pane, highlight/select Nullsoft DirectShow Decoder (in_dshow.dll)
and click the "Configure" button.
In the next window that pops up, under where is says "Extension list (semicolon deliminated)", make absolutely certain that AVI is listed there. If it's not, then add it.
The default list is: MPG;MPEG;M2V;AVI
Mine personally reads: MPG;MPEG;M2V;AVI;FLV;M4V
But in either case above, AVI is included. Make sure it's included in your Nullsoft DirectShow Decoder config too.
Now the above should work providing that you did not UNcheck video support during Winamp FULL
setup (not Winamp Lite - no video support at all, as you can see when viewing the Winamp Releases features list