Advertising is a powerful business model, and is there any better medium than demos that are directly embedded inside your users' systems? Yes. Yes there are. That is actually a terrible idea. Why would you do that? Oh. Right. Money. You know what? Fine. If it lowers the cost of commodity devices, then it is not entirely horrendous. Advanced users should have some method of opting-out, though.
Sure enough, Microsoft might have made that possible.
Paul Thurrott has compiled a little article that describes what you need to do to get clean installation media for your device. The procedure is fairly simple for Windows 8.1, although the Digital River download links for Windows 7 are good to know. The post is really more of a checklist to make sure you have your ducks in a row before attempting. Probably the most important advice (besides BACK UP!) is, especially if this is your only internet-capable device, make sure you have functioning network drivers. Also, if you have Windows 8.1 with Bing… sorry, you're stuck. Also, sorry in general.
Otherwise? Congratulations! You're now an enthusiast. Actually enjoy Windows.