Outlook XP Calendar disappearing act

We are running an exchange server 2000 with stations that have Outlook XP. I've been having a problem with a few computers in particular. A Secretary emails appoinments to be accepted to 3-4 people. After a given amount of time, all of the appointments disappear from the calendar, nowhere to be found. I looked strenuously on the server but i found nothing. I was wondering where outlook stored it's calendar items on the server, and if anyone knows of something similar to this ever happening. Thanks.