I'm not sure of the exact details but it seems that on some installations of XP the "show desktop" icon is missing from the quick launch toolbar (next to the Start button). Most other applications are also available in the Start menus so it's pretty easy to restore them but not the Show Desktop icon.
The Techie Corner blog had the answer I was looking for - two different ways to restore the Show Desktop icon.
I preferred the second technique - throw the following command into Start ->Run:
regsvr32 /n /i:U shell32.dllYour show desktop icon should have instantly appeared in the quick launch toolbar!
Technorati Tags: Show Desktop, Quick Launch, Windows XP, TechieCorner, Andrew Beacock