I know I'll get roasted for this, but try Microsoft Security Essentials. I've used it for several years now, and its the best one I've ever used (including AVG, Norton, Avast, and others.
When Microsoft was just DOS it was great. Then Windows 3.1 was a real step down. After that things got worse and worse. But Windows 7 and its associated supporting software has been just fine. You MS haters should try them.
Once I learned that Microsoft Security Essentials works best when there are no other security programs installed, I got rid of all the others. No problems since and it updates automatically.
It works great.