We have sixteen Windows servers and ~200 Windows clients on our LAN. In the 7+ years I have worked here, we have had only two virus infections that went anywhere at all. One was on our MS Exchange server, which is managed by an incompentent contractor, so we couldn't have fixed it if we wanted to. The other was a Nimda outbreak that took all of three hours work with two admins to catch and eliminate, with no data loss.
Since that time, every machine on the network has an enterprise-level virus scanner running at all times, and we have not had a single infection.
If viruses and security are a problem on your Windows machines, you have hired an incompetent administrator.
We also have had our Windows machines virus free for the most part. We run Trend's Neat Suite, it is expensive but effective. We have on most days over 300 infected emails hit our Exchange Server. I have seen on high outbreak days numbers approaching 1,000. This is irritating, time consuming, and a constant concern. I am looking forward to the OSX Intel based servers coming out as we are switching our servers over to them at that point.