Okay, so you've got someone coming in to do a presentation, class or pep talk. They're bringing their material on their own laptop or media device of choice, and you want to have everything ready to rock.
You've got your room set up with an LCD projector, screen, PC and laptop video connection. Every thing's plumbed through a nice neat patch panel with USB inputs and connected to the in-room sound system. You're so golden, your tail's waggin'!
and then the guest arrives...
They plug in their laptop and things go well, until they get to a slide that calls out a video clip. On the screen, all you see is a black box where the video should be. Yikes, talk about a buzz kill.
On some machines, videos will play correctly in PowerPoint when using a single monitor, but will only show up as a black box when the system is driving both it's own monitor and an external one. The quick fix for this is to FN/F4 toggle your local screen off, then restart the video. A less crippling to the presenter, but slightly more time consuming fix, is to turn off video hardware acceleration.
Of course, the real solution is to update your video drivers or upgrade your ride, but these aren't going to help you in the here and now. Warning: DO NOT be the guy who starts updating stuff 2 minutes before, or worse, during, a presentation.
Here's a link to a great site for more in depth info on both the quick, and the correct, fixes: PPTools