I'm amazed no-one has mentioned Microsoft's free Security Essential anti-virus. My NOD32 antivirus ran out and was looking around for a free replacement, and the Microsoft offering is very good, very light on system resources and although scanning speed is a bit slower compared to AVG, etc, its fine on my lappy. The interface is very easy to navigate as well, and because it uses Windows Updates to download definitions, it integrates quite well within Windows.
