This is the only fix I know, and I've never figured out if it hurts any other performance - when running a slideshow or on my computer in general. So when I'm done with a presentation, I reset it.
On Windows XP, go to your computer desktop, right click and select Properties (or go into the Start Menu --> Control Panel --> Display). Go to the Settings tab, click Advanced in the lower right corner. Go to the Troubleshoot tab, slide the Hardware Acceleration down one notch. Hit apply. Try running your PowerPoint. If you still have a black hole, go back and move the slider down another notch, etc. Eventually you should see your video.
On Vista - I'm 98% sure the control panel is the same and you should be able to locate it through Control Panels or something from the right click - I just don't know what it's called.
And this accelerator setting applies to any version of PowerPoint.