At Startup looks for eUpdates when your Mac starts up, while At Shutdown checks when you shut the machine off. ITS recommends choosing At Specific Time . The default specific time interval is Monthly, and the time and date selected correspond to the current date and time. For example, if you're setting up your schedule at 2pm on the 28th of September, Virex will default to eUpdating on the 28th of each month at 2pm. Choose a frequency, date and time convenient to your needs.
Monthly checks are adequate.
Just remember to leave your Mac running if you schedule updates for odd hours. Once your schedule is configured, click Add . A sample eUpdate schedule. An Untitled event is added to the white pane at the top of the dialog. When you're finished naming, click Save .