Which version of Windows are you using?
If you are using XP, go to the Start Menu, "All Programs", "Accessories", and open "Notepad".
Type the following code in Notepad.
[Shell]
Command=2
IconFile=explorer.exe,3
[Taskbar]
Command=ToggleDesktop
Save the file to your desktop as "Show Desktop.scf".
Now, drag the "Show Desktop" file from your desktop to your Quick Launch toolbar by using your right mouse button.
Click "Move Here" in the menu that pops up.