If your “Show Desktop Icon” on the Quick Launch toolbar disappeared follow the steps given below:

  1. Click Start, click Run, type notepad in the Open box, and then click OK.
  2. Carefully copy/paste the following text into the Notepad window:
    [Shell]
    Command=2
    IconFile=explorer.exe,3
    [Taskbar]
    Command=ToggleDesktop
  3. On the File menu, click Save As, and then save the file to your desktop as “Show desktop.scf”. The Show desktop icon is created on your desktop.
  4. Click and then drag the Show desktop icon to your Quick Launch toolbar.

Links: http://support.microsoft.com/kb/190355