I feel like a little information would be helpful to some of you, heres my opinions on the top free anti-virus's.
1. Avast Anti-Virus
An extremely comprehensive program, yet takes quite a bit of resources. There are many features in this program, that make up for its extensive resource hoggishness, including it's web browser protection, integrated firewall, email protection, and ease of use. The interface is gorgeous, and easy for all levels of computer users. It even includes a "gaming-silent" mode, that I rarely use, that shuts off notifications and sounds that might interrupt your experience. It is free, but requires registration. Also, it is important to note that it updates daily, automatically, without bothering you.
2. AVG Free
An extremely popular program, AVG is simple, and effective. It has everything the average user requires, with a no-frills approach. I would suggest this for all, it is low-maintenance, and quite simple. As looks go, it looks as it is, simple. No registration, and great coverage, with no extra capabilities. My only complaint is spread out updates.
I tried this program for a few months, and have little to add to it, being that it is almost exactly like AVG. Great coverage, simple, and effective, with no extra abilities. Perfect for the average user. My only complaint was the annoying update pop-ups on every startup. But it updates far more often than AVG, but not as often as Avast.
I threw this in here for teh lulz. This is the biggest piece of crap I've ever seen, and people actually pay for it. It's a resource hog that pushes itself into every aspect of computing. Rid yourself of it, purge the evil! BURN IT AWAY!
In summation, for the average user, get AVG. If you want more ability and protection, get Avast, at the cost of a bit more protection.