Dissappearing Messages Outlook Exspress

Situation is:
Windows 2000, Outlook Exspress v5.5 or v6.

Messages in Inbox or any other message folder dissapear every x number of days. Current View is set to view all messages. No other setting that applies to deleting messages in Options.
This problem only happens with users that have combination of Win2000 + Outlook Exspress.
Users with Windows98 and Outlook Exspress are OK.