A varity of free antivirus programs.
A very good antivirus application. However, be aware that their is advertising pop-ups. Avira is light on system resources so it is a good option for older PC.
I use Malwarebytes as a supplement to my main antivirus program. Malwarebytes only runs on execution so it will not interfere with your antivirus program. This will help broaden your spectrum of coverage against viruses.
Free antivirus protection. This application is rather resource heavy and should be installed on newer PCs and laptops only.
Avast A popular antivirus application.
Microsoft Security Essentials is a good antivirus solution for home use. No longer runs under Windows XP.