Speaking as an insider ...
I can say that in our Trust most of our essential patient focussed systems are web based, e.g. 'Lorenzo' for patient notes; 'CAYDER' for real-time bed management, etc. As for all the office type tasks we run W7, but they could also just as easily be done through web based applications.
That leaves us with certain specialist administrative software (there may be non-Windows/web based alternatives); and device based software, but I would imagine most of that is non-Windows based anyway, but there will be notable exceptions.
Overall, there is plenty of potential for getting out from the Microsoft hegemony, but who is going to make the calculation that doing so is more cost effective than keeping with the devil we already know, and avoiding the monumental scream-fest if people have to cope with a particular screen no longer looking like it did before. We've just been through that shifting to the Lorenzo system. It all died down of course and the new is now normal, but the piteous wailings that ensued something to behold - some people just don't do change, except at the point of a gun or threat of dismissal, and even then they take it hard.