How can I restore the Show Desktop icon to the Quick Launch bar so I can quickly return to the Desktop?

Copy and paste the following lines into a Notepad file.

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

Save the file to your Desktop as Show Desktop.scf. Make sure the extension is .scf not .txt.
Next left click on the file you just created and drag it down to the Quick Launch bar and release.

For more detailed instructions covering all Windows OS version, see:
http://support.microsoft.com/?kbid=190355