Almost all computers these days are sold with a free trial of anti-virus software pre-installed. The problem with that is many people let it expire and never bother to pay to keep the updates or look for an alternative. This is a friendly reminder to keep your computer protected and it doesn't have to cost you a thing.
There are several anti-virus options from which to choose. Many ISPs, like Comcast or AT&T, offer a free version of popular anti-virus suites, such as McAfee or Norton. I have in the security section of software links a few anti-virus solutions that are legally free to use for home/personal use. AVG & Avast! have both had their share of praise for their free offerings. Microsoft's Security Essentials is relatively new. I personally have been a long-time fan of AVG. However, there are required updates that happen occasionally that are not as automated as they should be. This extra requirement of interaction leads some computers to be unprotected if the upgrade is not dealt with in a timely manner.
From a tech support standpoint, I want to recommend what requires the least amount of maintenance and interaction, something that runs quietly in the background with the least amount of impact on performance. At the time of writing this article, MS Security Essentials is my primary choice.