ANSWER: Dear Tom,
please find here something that could be useful for you:
http://officeone.mvps.org/transshow/transshow.html
TransparentShow can make a particular color of any presentation transparent.
The regions where the slides have the specified transparent color are replaced by the contents of your the desktop.
After loading TransparentShow addin in PowerPoint, select Slide Show -> Start Transparent Show... menu item. This dialog box enables you to select the Transparent color and start the slide show.
Make sure you have the display mode set to 16-bit or 24-bit color as the 32-bit color mode may not work with some video cards.
TransparentShow is supported on Microsoft PowerPoint 97, PowerPoint 2000, PowerPoint 2002 (XP) and PowerPoint 2003. It requires Windows 2000 or Windows XP.
Click on one of the following links to download TransparentShow:
TransparentShow11.exe (315 KB)
TransparentShow11.zip (286 KB)