Rob, this is the "Show Desktop" icon. On my xp machine it is located in C
Documents and Settings\User\Application Data\Microsoft\Internet Explorer\Quick Launch. On my ME machine it's on C
Windows.................................etc.
Type of file : Windows Explorer Command.
Description : Show Desktop.
Hopefully, you just deleted the shortcut and you should be able to locate the file and drag it back to the taskbar. Hope it works.
Scally