This is because you've got the 3vix codec installed, and the demo has run out.
For some reason QuickTime chooses this codec to play back some movies (MPEG is one) instead of it's own. Who knows why (not me!). The way around the problem is to either:
- Uninstall the codec (I think you can do this from System Preferences, but if not you'll need to get the original installer and "uninstall" it), or;
- Purchase it.