I like AVG. It's free, comprehensive and efficient. Once I clicked on an unfamiliar site, and *immediately* a horn blared and AVG popped up to say that a virus had invaded. It told me what to do, and I followed its instructions.

The scan found two viruses and removed them. It then did another scan and pronounced me clean.

I followed it up by scanning by another service and it too said my computer was clean.

That has been the only time anything was found but I scan regularly.