To recreate it :

Click Start->Run
In the “open” box, type “notepad” (without the quotes)
Click ok
Copy the following lines in your new notepad window :
[Shell]
Command=2
IconFile=explorer.exe,3
[Taskbar]
Command=ToggleDesktop

Click “File” in the notepad menu and select “Save As”
Save the file to your desktop as Show Desktop.scf
Drag the new icon from your desktop to your Quick Launch toobar.