Go to Tools > Options. Then click on the Other tab. In here you will find a checkbox named "Make Outlook the defailt program for E-mail, Contacts, and Calendar". If this is unchecked then Outlook should not assume control each time it is loaded.

In Outlook...

Go to Tools > Options. Then click on the Other tab. In here you will find a checkbox named "Make Outlook the default program for E-mail, Contacts, and Calendar". If this is unchecked then Outlook should not assume control each time it is loaded.