Why do you still need to pay for a commercial anti-virus/security system when there are now many free ones available?
Free anti-virus systems are not licenced to be used in commercial settings
Most of the best free anti-virus systems (such as Avast, AVG) are not licenced to be used in commercial businesses (or not for profits). The penalties for violation of the User Agreement can be harsh: 100 or 1000 times harsher than paying a few dollars a month for a commercial grade system.
Free anti-virus systems are fine for use in the home, as long as you regularly check their status (check they are running ok), and don't mind the bombardment of ads they display!
Free and non-commercial anti-virus systems cannot be managed easily
As part of our IT Support, we monitor the condition of every device on your network and part of that is using a 'managed' anti-virus systems which allows us to verify each device is up-to-date, resolve infections remotely and schedule scans to run at convenient times. Free and non-commercial anti-virus does not have this option. Anti-virus which cannot be managed could lead to a device which is unsecured (opening up the entire network to attack).
The built-in anti-virus in Windows 8 is very basic
The built-in anti-virus in Windows 8 called Windows Defender is very basic and only should be used as a 'stop-gap' measure in commercial operations. Its virus detection is not as good as other free anti-virus systems and, much poorer than commercial quality systems. Additionally it cannot be managed remotely (to make sure it's working).
So the best rule is, a small monthly fee to have a high end anti-virus and security solution (such as what we offer in add-on services) will pay for itself in overall system security and labour hours in no time because commercial systems are easier to Manage and provide overall better protection.