I use this http://www.webroot.com/En_US/consumer-products-antivirus.html
It has everything you need and is easy to use.
I recommend this to all my clients but I know that others have their personal preferences.
Whatever you decide to use, make sure you've got anti-spyware and anti-virus software as well.
Good luck
youd be better off purchasing a nice sonicwall appliance instead of crap software. Unless you are being stupid, crusing porn constantly and trying to dowload shareware kazza bearfuck whatever its called now a days you really are fine, no one is trying to break into your less than stellar cable connection for anything usefull.
Now if you are running behind some 100mbit lines let me know, i can find some uses for your machines
Good call Michelin, i forget that most home users don't know what they are asking. It didn't occur to me that someone would be mistaking a firewall with virus scan or spyware scanning software. Im on the corporate side of things so someone says firewall, i recommend a sonicwall appliance