I meant like a gateway device, plugged in behind a router. Not at an ISP level, a customer network level. Just like what untangle does.
I know it won't have a perfect detection rate, just like any other A/V product, but the point is so that you don't have to install software on every computer in the entire network and manage them all.
That said, I guess it wouldn't be very good if something got past the appliance and got on one or all of the machines, then you'd have to install some sort of a/v anyway. But, at least you can stop most of it before it got onto your network in the first place.