PCLive: free security suite for Windows
PCLive Security includes the ClamAV antivirus engine, a bi-directional firewall, and a pop-up and adware blocker.
There's also a paid version that includes technical support, hard drive maintenance, and file optimization features for $4.95 per month.
PCLive Security runs on Windows 2000/XP/Vista machines. On the downside, the installation process takes forever, and once PCLive Security is up and running it seems to be a bit of a resource hog. Oh yeah, and it will try to uninstall any other anti-virus software on your PC when you run the installer.