I can only speak from personal experience.
Here lately I have been seeing an increase in the number of PC’s being infected with malware. Particularly the type that show windows pop ups that say the machine is infected and for X dollars they will sell you the software to remove the infection. All of these machines were running Norton. Some are here at work and others belong to friends that have asked me for help. All but two of these machines had to be wiped out and reformatted to get rid of the malware.
At home I have been running AVG free along with spybot search and destroy for over two years now. I and my family spend a great deal of time on the internet. I have never had an infection that either AVG or spybot did not catch before it’s did damage. Just remember to use the immunize feature in spybot.