I guess there is an example in the code archive to grab the desktop as an image. All you need to do is using GetSystemMetrics to get the Desktop Resolution, then use the same resolution with Graphics or Graphics3D, then continously draw the grabbed desktop screenshot as the background of your app.
check this out:
http://www.blitzbasic.com/codearcs/codearcs.php?code=654
check this out:
http://www.blitzbasic.com/codearcs/codearcs.php?code=654