SkywayTraveler

I am using Outlook 2003 for everything EXCEPT email, which I am using Yahoo mail. I have the box unchecked for Outlook to be my default email provider. Scenario: I shut down my computer after using Outlook calendar and Yahoo email (and other things). The next day when I reboot, I get a popup from Yahoo stating that my default email has been changed to Outlook. I then restore it back to Yahoo email, and go about my work. Now, the scenario above is repeated. How can I stop Microsoft was taking control of my email and stop these popups?