Have something that copies the desktop (I done that one time, it's easy), then when they click, prompt them "Format C: ?" with a yes or no, and no matter which they click, have it appear to start formatting. If they press any key or click or whatever, have it pause and ask them "Abort Format?" with yes or no, and then have it "Completing Format..." or whatever no matter which they click.
Then when all is done, tell them "Format Complete, Windows/Linux/Whatever is ready for re-install"
Or have a "Virus Installing" with a skull and crossbones image with... stuff like "Bank information detected, saving remotely" and then something about "Erasing <user folder>..." you could easily get the name of the user folder to make it appear more authentic.
If you could get the desktop wallpaper location and filename, you could even change the image so it appears like all the icons vanish from the desktop.
For something simpler, make a desktop wallpaper that has icon images on it, set that as wallpaper and see if they try to access the fake icons.