AFAIK if LW's media player does not have the correct codec to play the file it will seek codecs installed on your computer or even use an alternative player if that format is not supported.
You could always try the LW beta which has some improvements with the media player try LimeWire 5.5.8 - Lime Wire

However I'd suggest you update your video codecs. http://www.gnutellaforums.com/353926-post2.html and http://www.gnutellaforums.com/349710-post2.html