What on EARTH are you doing (or the clients) to get infected.
I've been using Windows since Windows/286 (anybody remember that far back) and in my whole time I have NEVER had a computer virus -- and in my "shadier past" I have been known to have used some torrent sites.
I think people can get caught out on any website these days, even "reputable" ones.
NBC website hacked and distributes malware – here’s what happened | Naked Security
(Admittedly that's a post on an AV Developer's site
but I've no reason to doubt it.)
With the advent of a decent built in protection in Windows 8 (FOR HOME COMPUTERS -- Corporates are another issue entirely)
I think this is a big step forward in W8; I don't think Microsoft get enough credit for it. It helps everybody because, for instance, fewer computers are in botnets sending out dodgy emails.
Personally I use Firefox with NoScript for browsing. It's a bit of a pain to enable scripting on each website you
want to see, but it's a good way to catch new exploits that the AV doesn't pick up. I've an idea that latest versions of Firefox have a similar capability built-in.