Some lost appointments

We are running Exchange server 2000 with 6 workstations running Windows XP
with outlook 2000. Recently, we have been losing misc appointments. It only
seems to happen when I go into calendar and make a change to an appointment
(it can be any kind of change, even just adding a note). The next day, some
of the appointments are gone. There does not seem to be any common
denominator, some but not all appointments with changes are missing, no new
appointments are lost and it is not all appointments with changes., just some.

Please help? I have scanned all computers for viruses and spyware,
diskclean on all computers, updated windows on all as well.