The show desktop button is available to all users in Windows XP and Windows 2k, if you delete it from one profile it still is under another
Assuming you have more than one user on the pc
(or you can create a quick account, logon then logout, then logon as your account that is missing the file)
once you log back on go to the follwoing folder in ANOTHER user name, not the one you are currently logged in as:
C:\Documents and Settings\Administrator\Application Data\Microsoft\Internet Explorer\Quick Launch
Under this folder just copy the "Show desktop" shortcut and paste it into the current users C:\Documents and Settings\Administrator\Application Data\Microsoft\Internet Explorer\Quick Launch folder
This should fix it!