I would say that if the ONLY place you go with Windows is to that one program and it does just that one update, then you can probably get away with no AV. Just backup before each time you go online from Windows. As I said, I have one program that needs Internet to operate under Windows, so I back it up before I go online so that IF it does have some attack, I can delete that version and roll back to the previous, un-infected image. But be careful if you do that to make sure that you really stick to that discipline.