oops.. sorry. After Deadeye posted, your question makes a lot of sense. The big problem I see is that you want the user to destroy the desktop and I am assuming this is the desktop as Windows first boots. He/she might have several windows open which even if you automated a screenshot using Python or something else you wouldn't get the "real" desktop.
One way to do it, arguably somewhat ghetto, is to have the player take the screenshot. You could flush the clipboard using the System action at the start of the layout, pop up a message box asking for the user to "goto the desktop" and take a screenshot, then save the image using the Image Manipulator. Then just use a while loop to confirm they actually saved it...before starting game.
Here is an example: http://www.box.net/shared/pi0gbddbdc
You could also do this with the PyQt4 module. Unfortunately, I can't get Construct to place nice with this even though it works flawlessly outside. You have to download the PyQt4 binary installer, Sip and MinGW to run this.
from PyQt4.QtGui import QPixmap, QApplication
app = QApplication(sys.argv)