C:\Documents and Settings\%username%\Application Data\Microsoft\Internet Explorer\Quick Launch
Check under that location, but use "default user" for %username% or you could try "All Users"
Default user is used to create the first profile.. That's the location of where the show desktop file is. It's somewhere in the C:\Windows directory.. but it has a different name and icon if I remeber.
or if you wanted to get nerdy..
Open Notepad and enter the following text:
[Shell]
Command=2
IconFile=explorer.exe,3
[Taskbar]
Command=ToggleDesktop
Save the new file as Show Desktop.scf then drag and drop the icon on the Quick Launch bar or whatever location you want the shortcut to appear.