The database of comets in Starry Night is updated regularly to reflect comets currently near enough and bright enough to be observable. No attempt is made to include all comets, past and present, because that would slow down the performance of the software. You can, of course, add any comets you want to the database by entering their orbital elements "New Comet Orbiting Sun" under the File menu. Comet elements may be obtained from the Minor Planet Center.
Starry Night Software Support