I've been writing a good deal about the OS X and malware for the Mac, which is much newer than the same territory on the PC side. However, that doesn't mean that PC users don't need the same attention. I've seen and used several free, home antivirus options out there. There are three that are top-of-mind when it comes to this sort of thing.
AVG: AVG Free
The AVG is a fine product to provide free, basic virus and anti-spyware protection. You don't get a great deal of nagging to upgrade, and it does automatic updates. But, you need to register to get their free code for a year, then re-register each ear to continue to get the free service.
Avast!: avast! be free
The Avast! product does a fine job as well, but one of the things that bothers me about it is that while it does it's nightly updates and scans, when it's done, it boldly announces it. So, if you leave your PC speakers turned on, and heave help you turned up, at three o'clock in the morning, you may be awakened to your computer announcing that it's virus-free.
Microsoft: Microsoft Security Essentials
This is my pick of these options. There's no need to register every year, no loud announcements that you're virus-free. It doesn't feel like it has a large memory footprint in your system, and typically it does it's job and moves on through. It updates with Windows, it's color coded so you'll know when you need to give it some attention for a more immediate update or that it's trapped something in quarantine and wants to delete it. However, one of the best things about it is that you can use it for Small Business as well as home. Microsoft happily allows you to use it on up to 10 PC's in your small office. And for small, even tiny businesses, that's a major boon.
Reguardless of which of these you go with, don't leave your computer unprotected. With something, you're far better off than nothing!
PS - Don't be fooled by products that may have made their way onto your system like McAfee Security Scanner or Norton Security Scanner, these products are a 'lite' application and their job is just to scan, not to fix. They want you to upgrade to their plus, or full version of antivirus software.
PPS - Don't run two antivirus protection software products at the same time! They will slow your system down signifigantly. They'll battle one another back and forth for whose scanning what, eat up system resources and you'll wish your computer only laggy. Pick one and stick with it.