Windows Registry key for "check whether IE is the default browser"?

In IE under Tools-> Internet Options -> Programs there's a checkbox labelled "Tell me if Internet Explorer is not the default web browser." What's the registry key that corresponds to this checkbox? The reason I ask is that I want to suppress this check programmatically.

Clarification: I don't want to find out the default browser, I want to stop IE checking if it is the default browser when it starts up.