I've had the "LaunchBase is already open" message box every time I install something too, but clicking OK on that little prompt before doing anything with the installer seems to make whatever it is install correctly... have you tried that? Or do you mean the message won't go away?
Edit: The message box may be out of focus behind the installer window. Just click on its button in the taskbar to bring to the front.
Edit: The message box may be out of focus behind the installer window. Just click on its button in the taskbar to bring to the front.