I've used AVG Antivirus free for a number of years without complaint, but recently I've had annoying constant popups asking me to reboot and updates failed while uploading. (These seem to be common complaints). Yesterday, I gave up, uninstalled it and started researching other free antivirus programs.
I've found the free programs more effective than others that require payment.
PC Magazine has several reviews on antivirus software and also free antivirsus software reviews.
Panda looks to be at the top of the list.
Let me know in the comments section if you have used free antivirus software that you've found effective.