Movies, directX or openGL games mostly run in hardware accelerated mode to reach maximum performance. This hardware accelerated mode is beyond the reach of any application. When you try to grab the image of a window running in hardware accelerated mode, you will get a black rectangle in the content area. The only way to grab an image from this source is to temporarily set the hardware acceleration to minimum.
To change the hardware acceleration settings, right-click on the empty part of the desktop. Click on Properties. The Display Properties will be shown. Click on the Settings tab, then click on the "Advanced" button. The graphic card dialog will appear, click on Troubleshoot tab. In the middle there should be a trackbar for hardware acceleration. Usually it is set to maximum (remember the position if it is not the maximum, someone may have set it there for a reason). Pull this trackbar to the minimum (none) or one after minimum. Click on the "Apply" button. The screen will turn black for a while, then revert to normal.
Although it is possible, avoid capturing fullscreen movie with a fullscreen capture option. For some slow computers, this task will be really heavy as the movie player will busy decompressing the movie while AviScreen is busy compressing a large video and writing it into a file. In this situation the shortcut key may stop working.