Anti-virus is a waste of time, money, and resources. It has been proven time and time again. At best, it will catch 1+ year old viruses, which isn't going to help anything. All of the new malware, like ransomware, just march straight through AV software.
http://www.theguardian.com/technolog...xpert-symantec
The best defence is to stop downloading free crap as it is typically loaded with malware. That, and don't accept attachments via email. Ever.