Try AVG Free. I use it on all of my computers (desktops, laptops) and it works great. It has one of the smallest footprints of any AV and does realtime as well as scheduled scans. It also plugs into a lot of software such as browsers, email clients, etc.
They update the virus signature database daily and even more quickly when a new, serious threat is discovered.
I have used it for about three years now and, knock on wood, have not been attacked since. It detects spyware, malware, viruses, trojans, whatever and the price is right--FREE!!!