My experience and the one from Imperva are not that far away:
“Antivirus software is now so ineffective at detecting new malware threats most enterprises are probably wasting their money buying it, an analysis by security firm Imperva has concluded.” – Source: Techworld
So why not choose a freeware Antivirus Solutions for your Business (all sizes) Computers?
With Windows 8 and the appendant Windows Defender, an full Antivirus Solutions for Windows is already in place. –> http://en.wikipedia.org/wiki/Windows_Defender
But Windows XP, Vista and 7 still need an Antivirus Solution!
There are plenty so called Free Antivirus Solutions out there, but they are mostly free for private use only –> http://en.wikipedia.org/wiki/Comparison_of_antivirus_software
Therefore, I made some research and found 3 (three) FREE for business use Antivirus Solutions:
Is it free for business users too?
Yes. However, enterprises looking to implement Internet Security on large networks of workstations would benefit from the centralized management capabilities of Comodo Endpoint Security Manager (CESM).
Q4. Is “Immunet 3.0, powered by ClamAV” free for commercial use?
A4. Yup, Immunet 3.0, powered by ClamAV is free for commercial use. Use it wherever you want to, home, office, servers, education facilities, non-profits, etc, etc. In fact the more places you installed it and the more systems it’s running on, the better its community-based detection will perform. – Source: ClamAV
PC Tools – is it free in business use?…The short answer is Yes. – Source: PCTools Forum Thread
As a BUSINESS: Despite you need antivirus software because compliance regimes mandate it, you are free to use a FREEWARE Antivirus Solution to keep your money for your VISIONS and business GROWTH or for a better IDS/IPS/additional security layers like the one from ZeroVulnerabilityLabs to protection your systems against advanced persistent threats.
BTW: Microsoft Security Essentials is free for Small Businesses (up to 10 systems) only.