Keep loosing my old Emails from outlook

Our Email setup is Exchange 2K as Server and Outlook 2002 as client. In many times, when I log on, I find that I can only see Emails only one or two days old, and non of all previous ones. All Network settings seem to be OK, and this happens to few users only on the network (5 out of 200).
Emails are set to stay on server (Exchange Mailbox) and NOT Personal Folders (.PST), which I think any change in Profile or Home Directory should NOT affect stored messages.

I would appreciate it if any one could explain to me why it is happening and how to solve this problem.