2 EMail Programs are flip-flopping

I have a user with an XP-Pro PC with MS-Outlook (2002)installed. He has a signature arranged when he sends Outlook messages.
Sometimes he recieves text mails from other clients with email links. When he clicks on the link suddenly Outlook Express opens up! OK, fine, so the user writes his message in the Outlook Express window and sends it off. Now he complains that his signature (in Outlook) is not attached to the Outlook Express message which he sent off. I’ve tried to find a way to work around it, but no luck so far.
Any ideas?
Thanks

Kirk