A virus can come from files, e-mails, web pages, or even devices you plug in (like thumbdrives or printers) and destroy your files or your computer once they get in. An anti-virus is software designed to detect and prevent that from happening.
For them to work properly, they must download software updates on an almost daily basis, so it is best to use free anti-virus products or keep the subscriptions current. Also note that you can't install multiple anti-virus products simultaneously as they will interfere with each other.
Our currently recommended anti-virus product is AVG:
I'd used Norton products for years because I got them free from working in retail or attending a school with a special license). Now that those days are over, I've had to look at paying for the anti-virus and I've begun to wonder if it's worth it.
Norton software is a notorious hog of system resources and can drastically slow down your computer's start-up time. Switching to free alternatives is a big help.
The newest version of AVG works very well and also includes a feature that automatically checks your search engine results for malicious code. A site with a green check next to it is considered safe and anything else should be treated cautiously.
When you got to the site, remember to be careful to get the free for home use version and not the "Free trial" of any of their other versions. Also be cautious during the install process to uncheck any options for add-ons like Yahoo! Toolbar or such. Other than that, it should be a pretty straight-forward process.