Sometimes people loose this icon. It can accidentally be deleted or you may loose it for some other reason, but the bottom line is : only few people know how to restore it.
That's because it is not an ordinary shortcut.
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 and select "Move here"
If you can't move the new Desktop icon to your Quick Launch toolbar for whatever reason, you can also try the following technique :
Right-click the new show desktop icon on your desktop and select copy
Open Windows Explorer and navigate to
C:\Documents and Settings\<Username>\Application Data\Microsoft\Internet Explorer\Quick Launch (replacing <username> with the username for which you want to restore the show desktop icon)
In the right pane of the explorer window, right-click an empty spot and select "paste"