Create a text file in the location of your choice, but I'd recommend you put it in "C:\Documents and Settings\USERNAME\Application Data\Microsoft\Internet Explorer\Quick Launch" (without the quotes).
Open the text file and paste in the following text as it appears on the next 5 lines including square brackets:
[Shell]
Command=2
IconFile=explorer.exe,3
[Taskbar]
Command=ToggleDesktop
Save the file as Show Desktop.scf and you should have the button you want.
Bloody hell !!. Use windows explorer or My Computer and navigate to C:\windows\system\show desktop.scf Highlight the show desktop icon and right click on it - select 'send to' and 'desktop (create shortcut)'. You have now got a "Show Desktop" icon on your desktop. Drag a copy of the icon to the task bar and youve got it.