www.avast.com go to downloads and select the AVAST Pro Home version, it installs very easily and is a nice program, updates definitions FREQUENTLY which is why I like it. Also a lot of people use AVG which you can download
Here I guess it's a choice of preference seing how they are both common, but I can personally say I'm happy with Avast, I uninstalled Norton and use avast only with ZoneAlarm's firewall, which you can get
herehope this helps, and GL,