I actually just had the same problem this morning, Windows Support answers it rather well.
Windows Support Site page- http://support.microsoft.com/kb/190355
That page says-
To re-create the Show desktop icon yourself, follow these steps:
Click Start, click Run, type notepad in the Open box, and then click OK.
Carefully copy and then paste the following text into the Notepad window:
On the File menu, click Save As, and then save the file to your desktop as "Show desktop.scf". The Show desktop icon is created on your desktop.
Click and then drag the Show desktop icon to your Quick Launch toolbar.
Worked wonders for me, hope it works for you.