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?

There should be a setting to never allow Outlook to check or change the default email. Check in the mail settings of the main settings group.