i have been using AVG 9 Free since it was released late 2009 and have so far have had no problems with it. It has an easy user inetrface which allows me to control my use of the programme. I have configured it to scan once a week and it scans all incoming and outgoing mail as well as internet sites visited. I know that it is working because I have several pop ups warning of an unsafe page when googling. I have read reviews in the past about the reliability of AVG but I have never had any problems and I have used various releases since the Dark Ages (well that what it feels like!). I also use Windows Defender alongside it which can sometimes be knocked off after AVG updates, but as I am notified it is easy enough to reactivate.
This is one of the best if not the best free antivirus programme that I have used (others include Panda and Nortons that came preinstalled), and I feel it offers high level protection. Its free and it works!