ClamWin is a good open source antivirus program for Windows (the Unix version is called Clam AV, the engine behind Clam Win).
From my tests it has a nice detection rate. It has automatic downloads, shell integration (where you can use its functions through right-click). It also has plug ins for email verification.
The bad thing about ClamWin is that it doesn’t have (yet) a real-time scanner so you’re left at manually run system checks. But, this should be solved soon with version 1.0.
I have ClamWin and Clam AV installed on my Linux distro and Windows respectively. But… since some functionalities of the program aren’t stable yet, I’m left with no option but to use a freeware proprietary application as well as ClamWin.