when you installed winamp in the first place, what option did you chose?
the 'lite' option doesn't include video playback, nor any codecs unsurprisingly.
in fact, investigating further, you shouldn't need external codecs at all.
if you haven't tried already, do a complete uninstall, download it
again, then reinstall using the 'full' option.
if it still fails to work, then i'm stumped, and all i can suggest then is to use another media player for your movies. vlc
is very popular, and also doesn't need any external codecs.
*edit* after reading your last post: for most skins, the visualisation window doubles up as the video playback area too.