Windows Mail And Thunderbird Conflict

After installing Office 2003 on a Dell 1330 with Vista Home Premium, the default mail tool changed, of course, from Thunderbird to Outlook. I uninstalled Outlook, and the default email tool changed to Windows Mail. I have tried using the Default Programs control panel and other means to change the default email tool back to Thunderbird, to no avail. Each time it starts it asks me whether I want Thunderbird to be the default. Each time I click yes, but the setting doesn't hold.

I have gone through all the usual steps to define Thunderbird as the default. In Windows Mail, the option to change it so that it is not the default is mysteriously grayed out. how to get around this? Windows Mail seems more like a virus to me than a legitimate piece of software. It doesn't even show up in the list of Windows services, so I can't even turn it off! What's so weird about this is that it all worked fine until I installed Office 2003 ... now, I can't change the default mail tool.