I know of no way to make an appt default to private. Maybe someone else will
be able to tell you.
However, in addition to the default Microsoft Outlook Calendar, you can
create other Outlook calendars. For example, you can create a calendar for
your personal appointments. You cannot share the additional calendars with
other people. You can share your default Outlook Calendar, for business.
On the Go menu, click Folder List.
In the Folder List, in the Navigation Pane, right-click the Calendar folder,
and then click New Folder.
In the Name box, type a name for the new calendar, such as Personal Calendar.