These things are a bit of a minefield, & I'm not a video expert. I'm only guessing (and may be wrong), but I suspect your Quicktime on the Mac has encoded with some kind of MPEG-4 that that the playback machine does not have. I'm not sure from what you say whether the target is Windows or OS X.
The best utility that I know of for troubleshooting this kind of thing (Windows only) is Gspot
GSpot Codec Information Appliance which (on Windows) will show you what you have installed, and also for a specific video file, what is actually required to play it.
On OS X (and Windows), VLC player will give you quite a lot of information (somewhere in the media stream / information settings; I don't currently have a copy to hand to check).
Have a look in the settings for Avid Xpress (I've never used it) & see what it's encoding with. If it's H264, for example, then DivX is a good candidate if the target is Windows.
Sorry to be vague but that's about the limit of my experience.