Your Internet Service Provider (ISP) generally provides free antivirus software for their subscribers. Simply login to your account and download the provided antivirus protection for your computer. In most cases antivirus software is available for both Mac and PC,
Comcast offers virus protection through Norton 360: http://xfinity.comcast.net/constantguard/CGAPP/Norton
AT&T Uverse offers McAfee antivirus application: http://www.att.com/esupport/article.jsp?sid=KB402441#fbid=agDt20lAjHo
You can also call the customer assistance number for your service provider and ask them what antivirus solution they offer and directions on how to access it.
Here is also some reliable information regarding antivirus software
Samuel Merritt University Helpdesk