Why? I've only ever had a virus once, when I was naive and used LimeWire, and that was just adware rather than anything malicious. So long as you use a browser with a good safety record (= not IE) and don't run random EXE files without checking where they came from, I think you should be fine.
In my experience of AVs, the situation is "free, light on resources, good detection rate: choose two". I'd really rather just browse sensibly.
I had a year's Kaspersky subscription (Uni practically made us use AV when we started, it was against network rules not to.) Admittedly it seemed like a pretty good app, but it never actually detected any viruses, so I decided it wasn't worth renewing it.