I saw this link on the anti-virus rants blog: Catalyst Conversation Starter: The High Cost of "Freeware". Given that I've just posted two tutorials on how to install and configure free (for home use), I wanted to share my experience:
One of the products the paper specifically mentions is AVG. I've been recommending, installing an maintaining the free version of AVG on several computers for friends, relatives and family. In this entire time the only advertisement I've seen was in the AVG 8 control center. No popups, no messages, no nothing. Now, your mileage might wary, but I've been very satisfied with AVG (for example some time back I've tried a free version of AntiVir, which tried to upsell me at every update). Also, in my opinion, it's not the "software" which matters the most, but the configuration settings you use. I've seen well-configured free products be very light and useful and poorly configured commercial packages eat up 100% of the system resources on a quad-core machine, while blocking access to the network and not giving any information about it.
My end conclusion would be: the most important thing is not what you use, but how you use it.