It's no different than human viruses, there are no guarantees. Personally I just use Microsoft Security Essentials and the built-in Windows firewall, never had a virus (at least not one I knew about ). If you're going to use these types of products, just make sure they provide "active" virus protection. IOW, they check files coming into your systems in real-time, not just after the infection has occurred and found only after a scan (most probably do, but it’s not a bad idea to confirm). Of course, this can reduce your network performance to some degree, but it’s a price you may have to pay if you want maximum protection.