AVG Free is a very popular and reliable anti-virus program (you can also purchase a paid version with more features. AVG Free has released a new version, so you can't get security/virus updates for the old version anymore.

You must download this new version, but they make the link for the Free version a little difficult to find. It almost appears that you have to pay for it, but if you look closely, you'll see the link tucked at the bottom of the page.

Here is where you go to download the new version:

http://www.grisoft.com/ww.93825.tpl-mcr1

Because of so many users downloading the new AVG Free version, it might be slow downloading the update, and you might have to try to download a few times.