if it's avi, it can't be rm or qt.
with gspot you can find out what codec it uses, what you have installed, and even test them. although the test sometimes says a codec works even if it doesn't. if it shows you have the right codec, you might need to re-install that, because sometimes some codec packs confict and break each other...
http://www.headbands.com/gspot/
using something other than mediaplayer might work too. like bsplayer (my favourite), media player 6.4 (last mp that ms didn't screw up) or 'media player classic' (not ms, but people continued from mp 6.4 after ms removed some essential functionality from it). bot media players are quite bare bones, but they work.