Ok, I think I know what happened.
The icon on your quick launch (I'm assuming by button you mean the icon on the quick launch, not teh right click option) is a shortcut and you deleted the file containing the windows explorer command (or it was lost somehow).
To fix this you have two options.
A) I just made and uploaded this command so you can download it.
or
B) I can show you how to do it (in case you are worried about what you are downloading, it's very simple).
To download just do this:
click here and save this to your desktop
After saving to your desktop click on it and drag it to your quick launch bar.
I only tested it on XP and while I think the windows commands are backward compatible I may be wrong. You can test this by running the file (click open not save when clicking the link).
You can then move the file you downloaded to any location on your computer (on the desktop the "show desktop" icon is useless) but make sure to update the shortcut in the quick launch bar.
If you are wary about downloading my file you can do what I did to make it:
This is the Windows command (it should reference the correct icon as well):
Code:[Shell]
Command=2
IconFile=explorer.exe,3
[Taskbar]
Command=ToggleDesktop
Paste that into notepad and save it as: Show Desktop.scf
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