AVG Anti-Virus is one of the best free anti-virus programs on the net, and supports features like automatic e-mail scanner, scheduled tests, free and automatic updates, and automatic healing of infected files.
I use this Utility to scan games and my emails for viruses, and it immediately notifies me when it finds a virus. It works with Outlook and other email clients to find viruses right when they are received. The only limitations to this free edition is the basic interface, no advanced scheduling of tests, no creation of your own tests, and no technical support.
But those don't matter because it's really easy to use and runs in the background when you start your computer. Definitely worth downloading, and will save you lots of money. You should always scan every file you download!