Windows media player wont play video?

I recently filmed some things for a project at school, and I put them onto my computer. When I went to watch the video I can only hear it. I have windows xp, and downloaded windows media player 11 for it. I didn't hear any sound until I downloaded something that filtered it to play AC3 audio files. The camera was weird too though, when I put the pictures on the computer I had to go back to a different file thing to get the video. If all else fails I'll ask for the camera back to see if the video and audio are separate files, but is there anything I can try to do to get the video to play?

When I upgraded windows media player it changed all the files to mpeg.