Messages disappearing in OE on Win 2000

I have a client whose e-mails in custom folders and the inbox are periodically disappearing at seemingly random intervals. The custom folders also are removed and the inbox is reset with Welcome to Outlook Express message for the current date. Theclient is using Windows 2000 and Outlook Express 5.50 and McAfee Virus Scan. This client also has three other workstations with the same configuration, two of which have never experienced this problem and one which has not experienced it since updating the McAfee Application in December.