If you’re lazy for the 5 pages article… it says of avast:
Avast
Tired of dealing with bloated, overpriced security suites that bog down your system and cost an arm and a leg, when all you want is antivirus software? Then get Avast, a superb antivirus program that’s free for home and personal use. Because it’s a lean piece of software, it imposes a relatively light burden on system resources and RAM. Despite this, it kills viruses in their tracks and has plenty of extras, including live scanning to prevent viruses from infecting your PC in the first place. Avast can scan regular and Web-based e-mail for viruses, too, and it protects against instant messaging viruses, peer-to-peer dangers, and more.