The Show Desktop icon in the Quick Launch toolbar isn't a normal shortcut. If you accidentally delete it, you can't recreate it the way you would a shortcut to a program. Instead, launch Notepad and copy these lines:
[Shell] Command=2 IconFile=explorer.exe,3 [Taskbar] Command=ToggleDesktop
Save the file as 'Show Desktop.scf' in the folder 'C:\Documents and Settings\username\Application Data\Microsoft\InternetExplorer\Quick Launch' (Replace 'username' with your user account name).