Stating the obvious, the Start up dialog appears when the application starts. It allows the user to run one of the wizards, or to load a recent file.
To disable the Start up dialog, uncheck the 'Show On Start-up' box in the dialog, or in the 'Show Start-up dialog' check box on the User Interface page of the Options dialog.
To enable the Start up dialog, check the 'Show Start-up dialog' check box on the User Interface page of the Options dialog.