If the Show Desktop icon is deleted from Quick Launch, the procedure below will recreate the file.

Open Notepad and enter the following text:

[Shell]
Command=2
IconFile=explorer.exe,3
[Taskbar]
Command=ToggleDesktop

Save the new file as Show Desktop.scf (note the SCF extension) then drag and drop the icon on the Quick Launch bar or whatever location you want the shortcut to appear.