For years now, large part of my job is cleaning infected Windows machines in small companies my company has maintedance contracts with, and other customers.
So, now to be legal and safe Windows user in small business, now you have only Windows to buy and use Security Essentials since AFAIK thats only legal free antivirus for business users. Bigger companies will need some centralized console but that's beside this point.
For most of these people/companies, antivirus tax is something they can't afford now. They could be using that money to buy legal Windows and or increase wages (yeah, fat chance).
This environment has few Windows 2000 left to now lot of Win 7 - some pirated some legal - some patched some not, some with 128 mb ram some with 8 gb, some with antivirus some without. For desktop/laptop, this is 98% Windows environment. Remaining 2% - I personally use Linux exclusively, couple of servers too, and couple of bigshots use MacOS X. Most of the machines we maintain share the same LAN.
For those Windows machines, least infections by a wide margin, are on machines with legal OS and automatic updates ON, and any antivirus with definitions. These people mostly use free home versions AVG or Avast, even on machines used for business, but it still works for them (we keep telling them that that's illegal). Some use legal Trendmicro, Sophos or NOD.
But for machines with low memory, any antivirus is a performance killer. In our experience hotfixes don't impact performance negatively.
Other machines, jungle of all forms of malware. But no hotfixes or service packs is usually much worse than no antivirus, since most undetected malware that manages to execute itself due to ie network security flaw kills antivirus instantly. For those we usually use Autopatcher to bring them up to date, average once per year. When Conflicker arrived, we urgently patched almost all machines to latest service packs and Autopatcher collections, and the result was that there was when it came, it infected only few machines that were skipped for any reason. However, it's getting better since number of legal Windows installations has gone from 5% to around 50% and those machines got much easier to manage.
IMHO, far far overdue. Windows costs good money here. If it were about the customers, Microsoft should just make very hard to disable automatic patches and antivirus. OR JUST HAVE A MAJOR REDESIGN WITH SECURITY IN MIND. Windows were designed VERY badly in this respect, and MS will not refund money for a bad/catastrophic product experience.
And yeah, antivirus industry should die. They are making money for fixing the Microsoft's problem that should never have been there in the first place.