If that's the case that's fine, but then my question is why are these computers networked to any extranet source? It seems a natural conflict: you cannot update the system due to it being so important that it always works, yet we need to attach it to an outside network which allows risk of infection. In my opinion, if the computer HAS to be connected to a network that is accessible from outside, then it MUST be allowed to be updated with latest antivirus/protection updates.