"Show Desktop" is not a normal program and you can't just make a shortcut or reactivate it in a normal way. What you'll need to do is create a file in Notepad and save it to where Show Desktop should be.

Go to Start/Run and type in "notepad" (without the quotes) then type in these lines:

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

Next, save the file with the name "Show Desktop.scf" in the folder "C:\Documents and Settings\username\Application Data\Microsoft\Internet Explorer\Quick Launch"