In My Computer or Windows Explorer, click the icon for the program you want to add, and drag it to the Quick Launch portion of the taskbar.
The icon for that program appears next to the other Quick Launch icons on the taskbar.
If the Quick Launch bar is not displayed, right-click an empty area on the taskbar, click Toolbars, and then click Quick Launch.
try finding the Show Desktop app somewhere else and copy and paste it into your directory. the quick launch folder can be found here:
C:\Documents and Settings\UserName\Application Data\Microsoft\Internet Explorer\Quick Launch
look under the Administrator folder (in place of UserName).