What is the best antivirus program?
This is a question that can only be answered in opinion vs fact. I have my preferences and recommendations as well as my "stay aways".
First and foremost let me mention that I use and recommend Webroot for all my clients. Webroot is lightweight and effective. The installation for Webroot is very thin and it has low overhead on the computer which means it doesn't slow down the system no matter how old.
I also like the built in AV that Microsoft has started to provide inside of the windows operating system. I find it to also be reliable and effective at finding and cleaning infections.
I stay away from the big brands like Norton, Symantec, McAfee, AVG etc... These giant security suites slow down even the fastest machines and do not seem to be any more effective than the less expensive products with much less overhead. I am not suggesting they are not effective against infections but I am suggesting they will effect your systems performance significantly.
Lastly I will mention that most infections are not viruses at all these days. The vast majority of today's infections are malware. Malware is best cleaned by free tools like Malwarebytes and SuperAntispyware as long as they are properly updated. Be safe out there and if it feels wrong then it probably is so don't go there, don't click it and certainly don't trust any pop ups.