This seems to be a hot topic because of the lack of description provided by the Live installation program.
To quote Len, one of the Windows Live MS employees:
Choice Guard is a piece of code that is responsible for setting the users homepage and search default if they chose those options during setup. Typically, this is a simple task of writing a value into a registry key.
However, several of our competitors install a "service" that is always running on your machine that prevents the registry key from being changed, even if that is what the user decided to do. So Choice Guard looks for the presence of those apps that might be preventing the user's choice from being carried out and works around their block to set the search default.
It only does this one time. If the user attempts to change it again, either manually or programmatically, they are free to do so. It turns out that in order to set the search default and homepage on the Firefox browser you need to install yourself as an add-on, which is why Choice Guard is appearing in the list.
