You are seriously not saying that a fully updated windows machine is naturally compromised. Windows machines (and servers) run tons of extremely sensitive data that is constantly under target and attack. This is not an operating systems issue this is a failure of OP who probably downloaded a fake wallet or most likely is just lying to get pitty donation funds.
Well i have seen it before, so that might be so. Even having the system secured and updated is not safe enough. Airgap might help your company server, but you are taking a giant risk. There are zero day exploits, there are "private" exploits now known by few discoverers but kept secret until needed. Usually those are not seen for widespread attacks, but can for targeted attacks. OP might have drawn someone's attention.
This is the fundamental problem of "security by obscurity". By hiding the code, is not like exploits don't exist, they do. And worst of it is they remain hidden for much longer than with open source.
It might be something simple like you say, but its always a good moment to call everyone's attention. Do not fool yourself thinking a well maintained Windows is ever good enough, it isn't. The thing didn't earn its bad fame for free. A better OS also needs maintaining, but its far easier to keep those secure than Windows ever will and this is a rock solid fact backed by decades of experience. Sure, you can make a living maintaining those (maybe thanks for being what it is), but that doesn't mean its any good. Internet is not run by Windows servers, they remain a minority for this very reason.