Restoring the Show Desktop icon to the Quick Launch bar is a little more complicated than I expected, but no big deal.
From Microsoft Support
To re-create the Show desktop icon yourself, follow these steps:
1. Click Start, click Run, type notepad in the Open box, and then click OK.
2. Carefully copy and then paste the following text into the Notepad window:
3. 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.
4. Click and then drag the Show desktop icon to your Quick Launch toolbar.
Information for advanced users
The Quick Launch toolbar uses the files in the following folder:
%userprofile%\Application Data\Microsoft\Internet Explorer\Quick Launch
Be sure the file created is .scf and not sct.txt or some other variation. The Microsoft Support link also has a link to fix the problem automatically using guided help, which I didn't use. If you've already upgraded to Win 7 you won't have to worry about making the mistake of deleting this option!
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Windows XP Pro