Across our machines and through personal experience, we have tested over a dozen different antivirus programs, most of them free, and have found our favorites. For the category of "Ease Of Use," nothing we tested compares to Microsoft Security Essentials. It is a wonderful set it and forget it program that does everything it needs to keep you safe. Just look for the Green House in your task tray and you know you are covered.
We recommend this program for anyone that isn't sure if they have antivirus already and don't want to be bothered with performing any tasks to ensure security.
For those that want to take hands on control of their antivirus security, check back next month when we will be announcing our favorite for the category of "Advanced Options."