Yes, Windows does have a hosts file, and this can be modified by a user with local admin rights. I expect content providers to eventually serve ads from their domain. They will move the targeting to the web server. We will still be tracked, and it will just be the server that asks the ad network for a targeted ad that is then served to you from the same domain. Right now, we can use the hosts file to manage much of the ad content, because they are served from a different domain from the content we want to see. We won't be able to do this once the HREF domain is the same for both. Ad blocking may then move to a filtering mechanism provided by a proxy server that analyzes and changes everything as the browser asks for page content. We may even subscribe to a proxy service in place of deploying our own.