I have used Sophos in the past provided by work (Free home licence for every office licence. ) It was excellent but had flakey update software when not stand alone.
AVG as recommended by many on here was fine. Used with zone alarm.
I now use Norman (
www.norman.com) for AV for its "Sandbox" which should stop new nasties before they get an update out and am lazily using the XP firewall behind my hardware firewall.
As for updates, once a day is the minimum. And practice "safe text" in any case.