The problem your suffering from could be the version of Quicktime you are currently using. Try updating to the latest version of the player via Apples website.

Another reason could be the codec that the video was originally compressed with isn't supported in quicktime, there are numerous codecs out there and it seems the PC boys keep developing more and more without bothering to check cross compatibility. It is a problem we Mac users have to put up with all the time I'm afraid, whereas most mac developements try to incorporate most users PC, Mac or otherwise.

Keep trucking.

Rich