This is an old one but good one,
Make sure the desktop is visible. Hit prt scrn on the keyboard.
Open up paint or some other image application.
Paste the image and save it.
Right click the desktop and select "arrange icons by"
uncheck "show desktop icons"
Right click the desktop and select "properties"
"Desktop" tab and change the background to the image you saved.
If they are not using the "auto hide" feature for the task bar this works especially well. Right click the task bar and check "auto hide taskbar"