MS Outlook taking over

I have a mystery here … 2 different laptops, both running same version Win XP Pro and MS Office. The mystery is this … why on one laptop MS Outlook wants to take control as default mail application rather than Outlook Express, even though OE has been set as default. As soon as Outlook is opened, it immediately assumes default mail application. The other laptop does not do this. The owner of these laptops uses Outlook for the calendar feature, not for sending/receiving emails.

Any suggestions for a fix to this?