It is fairly easy to restore the "Show Desktop" button that usually resides in the Quick Launch toolbar, if you have removed it.

Open notepad and paste the following:

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

Save the file as "Show Desktop.scf" in C:\Documents and Settings\<YOURNAME>\Application Data\Microsoft\Internet Explorer\Quick Launch where YOURNAME is the username of the account that is missing the icon.

What is over at http://faq.arstechnica.com/link.php?i=1825

Now I'm wondering, what are these .scf files and where are they documented? More importantly what else do they do?