I'm using Windows 7. Following your anser I've switched the video engine from QuickTime to Xuggle. Since I've switched to this option tracker works fine.
Tracker was freezing with every video I've loaded (I've tried my own videos and videos from the Internet of different lengths).
I'm glad to hear Tracker with Xuggle is working well for you :-)
I wonder if there is a problem with your QuickTime installation. It is highly unusual for users to report problems with QuickTime, and the fact that Xuggle works well indicates that it is not likely a Tracker problem at all. You might try uninstalling and reinstalling QuickTime...
