Keep the Show Desktop icon in it. If you had deleted it for some reason, then do a search for *.scf in the drive and if you find the Show Desktop.scf file, copy it to the Quicklaunch folder. If you do not find it, then create a new folder.

Open Notepad and type the following:

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

Choose Save as from the File menu. In the dialog box, change Save as type to All files. Give the filename as Show Desktop.scf. Make sure you save it in the folder you had created just then. Now, right click the taskbar and choose Toolbar -> New Toolbar. Here choose the folder where you had put the Show Desktop.scf file.