Must agree with slackrat77. I'm in PC repair and I'd say 70% of my work is removing PUPs even when my customer has antivirus installed. My main problem with Avast is all of the damn false positive it flags up. I've had it completely break a customer's installation of Sage accounts because it identified one of its vital files as a virus and deleted it. It also flags up one of the integral files in Java update 7 and prevents you installing it unless you disable Avast. Personally, I don't use any anti-virus and rely on common sense. If you're ever unsure of a file, upload it to https://www.virustotal.com/ and let it scan it.
Even the best free and paid for antivirus softwares don't tackle all the crap pup's that creep onto your machine. I spend most of my time on non hardware repairs running scans with adwcleaner, roguekiller, junkware removal tool, hitman pro, malwarebytes, spybot, decrapifier and ccleaner. People are far too click happy and am sick of seeing pricegong, delta search, conduit search and wajam slowing down customers machines. If there's stuff in your programs in control panel that you don't know what its for. Don't use or know know how it got there, run one or all of the above softwares!
Even if you do... some Government Dept/ Bank employee may wonga it... if the price is right ;)