My son was using Avast Antivirus but he was regularly being hit by a malware infection. I recommended he used Norton instead and now his PC stays clean. Unfortunately I didn't know about the high cost of subscription renewal at that time, so we both switched to using Comodo Antivirus (which is free). No malware is getting through with Comodo installed, so currently that's my recommendation to all who ask me what they should use.
Comodo freeware products are here: http://www.comodo.com/