Avast is good.
Kaspersky and NOD32 are better, but you have to pay.
The best thing about NOD32 is it's extremely low impact on system resources. There are more comprehensive protection programs out there with a scope that is larger than what you're at risk from via web browsers, but for keeping you safe from viruses, trojans, and the like, NOD32 is pretty bulletproof. The larger-scope antivirus programs almost always hog so many resources that you experience significant performance issues.