You didn't say which operating system you're using. That makes a difference.
I run Windows 7 on most of my machines. I've found Microsoft's Security Essentials to be an easy to use, effective product.
If you Google (or Bing) "free anti virus," you'll get lots of helpful information.
For Windows, you've got a lot of choices to go by. I tend to stick with the following list, with my more preferred programs listed at the top.
Avast! Free Edition
Panda Cloud AV
Microsoft Security Essentials
For Mac, Avast! is really the main one I'll recommend. For Linux, use ClamAV and don't run as root if you don't trust yourself 🙂
The most IMPORTANT part of computer security is to never sign on as an Administrator for usage. Create a Limited User account for you to use, and lock down the Administrator account with a password. Use that Administrator account for repairing the computer, updating the computer, and installing/removing the computer. It'll take some getting used to, but your computer will have a much better chance of being recoverable should you have a virus infection. All systems I build and set up for individuals are like this. Admin account is hidden except for in Safe Mode, and it is passworded. It won't save against an "ooops" but it will save against plenty of drive-by downloads which tend to be the nasty ones.