I posted yesterday about exporting a QT file from iMovie and emailing it to a PC user (thanks to those who responded), but said PC user claims it opens and “plays” in QT, but that the screen is just black and there is no sound.
Any idea what the problem could be?
Thanks. 
Do they have the appropriate codecs installed? They may have an older version of Quicktime that does not include the codecs you created the video with.
When you install the QuickTime player it gives you the option to install optional components. If you don’t have the right ones installed you may get a black screen when you try to play certain movies. I just had this happen over the weekend when I tried to play Coach Z’s video off the Strong Bad CD. Have your friend install the latest version of the player, choose a custom installation, and check the boxes for all the optional components.
You might want to make sure your friend has the latest version of QuickTime installed, too. Codecs from newer versions are sometimes incompatable with the older players.