Help!

I am having some issue with a video clip which is in mpeg4 format.

I have two video clips both in mpeg4 format. One of them plays fine when inserted from a powerpoint on Mac based system. The other video only gives me audio but does not show any output for the video. I have changed the video format to .avi but still nothing. Both clips play fine on Windows based system though. I am not sure what might be happening.

1. Is this a Codec issue?
2. If yes then should I install any specific codec package on my machine preferably a freeware?
3. But, I am not sure which codec am I missing?

Any insight will be helpful.

Thank you