I rather pay for the OS, instead of being subsidized by content that would try to be directing my eyes and workspace to something that isn't relevant and always need to show (either by popups at in-opportune times, a constantly shown banner that permanently takes up screen space, or prefixing my searches with ad-sponsored ones that may not be relevant).
Also, as much as I DO submit my system information and software errors to aid in feedback for OS support, I need to be assured that there is a clear, concise presentation of any personal information (files or usage otherwise) that would be submitted for such an ad service. (I.E. the terms of service and end user license agreement needs not to be in paragraphs of lawyer language)
This is BAD enough on free apps on phones, where the banner would appear taking up screen space or the banner being always there taking up screen space.
I do however, support a VERY LIMITED featured free Windows OS (think how Microsoft have and is still doing now with things like Visual Studio which is VERY good for enticing users for programming while still offering enough for capability for learning and making effective small programs). Call it Windows (number) Express Edition, and limit it to a certain memory size (4GB) and 32bit workings. Also, Aero would still work with the base level DirectX versions (9/10).
Wasn't there a quote somewhere about if something is pirated, might as well as it is something pirated that is made by us? This would solve any security issues that may arise.