Let's start with the easiest possibility -- Open your Inbox and click
View >Current View (or View > Arrange By >Current View in OL2003). Which
view is checked there? Does it say "Messages" or "Unread Messages in this
Folder"? If it's the latter, messages will "disappear" from the view as
soon as you read them so you really want to set it to display Messages.
Athena