I have been using ClamWin now for a year or so. I liked the fact that it has a smaller footprint.
I was getting ready to try AVG Free again, when I noticed in the license that it was only free for personal use, and specifically excluded non-profit businesses from the license.
I get excluding business, but why Non-Profits? Just feeling uncharitable?
ClamWin is a Sourceforge open source project.
Anyway, no infections using ClamWin, so I continue on.
In addition, I use:
Windows Firewall (installed by default on XP and up)
Firefox and Chrome
(Kim Kommando has her own set of recommendations for free security software)
On all but the lowest powered machines, I now install Microsoft's own Security Essentials . I find that it generally does not impede performance, as I find with many other av software, and so far has been smooth.