Avast Software(4 posts)
Except for a brief period when I made the mistake of using MSE, I've used the free version AVAST! for some time and it's worked just fine for me. I see no real need for the paid version unless the user of a PC hasn't the discipline to manually run periodic scans.
I've also used the free version of Zone Alarm's firewall for several years and have been happy with it. Their paid security suite doesn't rate all that high in reviews.
jmcguire is correct saying that you shouldn't run more than one AntiVirus progam at a time. Often (if not always), just having more than one installed will cause problems. Free programs that will play well with your AntiVirus (AV only, not a suite) include Malwarebytes (MBAM) and SuperAntiSpyware (SAS).