Internet Explorer keeps telling me that it is not my default browser

Every time I launch IE6 on my work computer it keeps telling me that it is not my default browser, which is very annoying:

Using Registry Monitor I was able to figure out that setting the STRING value HKCU\Software\Microsoft\Internet Explorer\Main\Check_Associations to “No” fixed the problem but changing this value is ignoring the problem.

The problem started when I installed Firefox to try something out and after uninstalling Firefox. I have to mention though that during the installation of Firefox 3.0.7 I unchecked the box to make Firefox the default browser but I guess that Firefox doesn’t care. So in order to fix this problem I had to restore the file associations for IE. This can be done through the Add or Remove Programs Control Panel applet:

By the way on my machine this button was missing due to Group Policy settings:

You can temporarily enable the button by setting the following registry value to 0:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Uninstall\NoChooseProgramsPage