Manual steps to re-create the Show Desktop icon on the Quick Launch toolbar
The Show Desktop icon is not an ordinary program shortcut. If the icon is deleted, the procedure for re-creating the icon is not obvious. To manually re-create the Show Desktop icon, follow these steps:
1. Click Start , click Run, type notepad, and then click OK.
2. In Notepad, type the following text on individual lines:
[Shell]
Command=2
IconFile=explorer.exe,3
[Taskbar]
Command=ToggleDesktop
3. On the File menu, click Save As, and then save the file to your desktop as Show Desktop.scf.
Note Notepad might automatically append a .txt extension to the file name. Remove this extension if present. The file name should be Show Desktop.scf.
Ref: http://support.microsoft.com/kb/190355