Operating systems, as I have said before, seem to allow more focus on applications, saving most of us from programming a personalized operating system. We have learned in this class that flaws are unavoidable, and nothing can be certain to work as required in every situation without fail. We must accept this, and move on, either just dealing with it, or to creating better alternatives. Windows seems nicer to me than what was before it, but I am sure nicer things can be developed. To find the right other is exciting.
Computers have nothing to do with purity.
Purity is doing what you really want. I don't really understand what this question is trying to ask, but I am confident that computers are abstractly antipure because they do what they are told, at least, that is all they try. So, in this way, while OSes may not be necessary, I think that they have definitely made computer use more accessible to the broader public. I don't think there is anything wrong with what Windows has created. Essentially, it meets the needs of a vast number of consumers and their computing needs by offering easily accessible, pre-packaged applications, and technical support to boot.