There is very little reason to pay for virus protection for your home computer. Some of the options include AVG Free and Blink and possibly a free suite from your Internet Service Provider.
My ISP, RoadRunner, is now offering a free Internet security suite from Computer Associates. I do not have personal experience with CA security software, but they do make good backup solutions that I have used extensively.
There you go. You have no excuse for not installing a good anti-virus protection if you are using a Windows computer. If you do not have RoadRunner check with your ISP to see if they do offer a free package.
Why would an ISP do this? Two reasons. The first is a little more to talk about in the marketing message. The second and most important reason is that Internet Service Providers have a financial interest in keeping your computer clear of malware and trojan horses. If any computer on the network that is pushing out SPAM or other unwanted traffic, they are footing the bill for the bandwidth. It is low-cost insurance to protect customers than it is to purchase more bandwidth and to increase their network capacity.