IIS 7 - 500 Internal Server Error trying to configure a default document in web.config


I am in the process of deploying an osCommerce site to a Windows 2008/IIS 7 machine as I write this (no point debating the merit of doing so, you really can't fight the management if they have taken a decision and you are not even a part of that particular decision making).

So they want a default document of let's say index.html for their website from where they want to take the user to osCommerce's main index.php based page. And I need to make the appropriate settings in web.config to have this going.