You could manually delete 1.8 - The default installation folder (under windows) is c:\program files\LimeWire - there'll probably be a 1.8 folder under there which you can safely delete.

You can delete any icons on your desktop or start menu that you don't want.

If you aren't confident about manually changing the registry - then don't worry about that - you'll be left with an uninstall option in Add/Remove Programs but there shouldn't be any other problems.

Mark