OUTLOOK 2003 - unable to add appoinment

[Peter]

Outlook 2003 suddently do not let me add appointments, the
calendar stops responding. I can select 1 day, 5 days, 7
days a.s.o., bud when I double-click on the calendar in
order to add an appointment, the calendar simply doesn't
respond. When I have logged off and on again it works, and
after a couple of hours the same sh... happens again

what can I do?

Thanks

Peter