I'm currently using ClamWin from http://www.clamwin.com/. It is a completely FREE virus scanning software for Windows platform, source code is downloadable too.

However, it is a bit slow while scanning. maybe they can improve it in future version.

I've tried AVG and found certain viruses are not detected. Furthermore they are not entirely FREE, especially if you want to use it personally in your company.

NOD is not free leh.