I would also recommend not using Windows. After all, the “security” that comes built-into Windows has been probable been breached 2-3 thousand times in the time it took you to read this sentence (I don’t have any real statistics, I just 2-3 thousand because it felt like a plausible number). I doubt there has been any software in the history of digital computing that has been breached more times than Windows. The gap between Windows and the second most-breached software is probably bigger than the entire grand canyon. Or something.
Anyway, a free (as in freedom) Unix system, such as GNU/Linux, would probably be easier to maintain and update than Windows, so it would make much more sense to me to pick a nice Unix system. Because of it modularity, Unix systems would almost certainly be more flexible than Windows, which means that portions of the OS that aren’t needed could be easily removed, allowing an administrator to be able to easily bend the OS to the bank’s will. You toss useless things out, so that you only have what you need. Added bonus: many cyber criminals won’t know how to work a Unix system, let alone how to breach it.