Method 1:

Click Start, Run and type the following command:

regsvr32 /n /i:U shell32.dll

The Show Desktop icon file should be available now in the Quick Launch folder. To access this folder:

Click Start, Run and type the following command:

%userprofile%\Application Data\Microsoft\Internet Explorer\Quick Launch

You should see restored Show Desktop icon. Drag and drop it to the Quick Launch Toolbar now.

Method 2:

 Click Start, Run, and type Notepad.exe. Copy the following contents to Notepad:

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

Save the file as "Show Desktop.scf" (with quotes, in order to prevent Notepad from appending .txt extension). Now, drag the file to the Quick Launch Toolbar.

Note: these two methods apply to Windows XP only.