There are two possibilities that I can think of. One is that there is insufficient bandwidth on the USB bus for both your camera and the Leap Motion device. Try pausing tracking from the Control Panel icon in the task bar. If the camera works with tracking is paused, then its a good bet that bandwidth is the problem. You could try different combinations of USB ports, though sometimes its difficult to know whether ports are on the same bus or not.
Another possibility is a driver conflict such as your camera driver trying to use the Leap device as a camera instead of your primary camera. Maybe check that your camera software isn't accidentally trying to use the Leap instead.
If neither of these help, please make a post in the Customer Support forum (or contact them directly). Our customer support technicians probably know more about specific driver issues and remedies than I do.