Avast is rubbish, all the "free" solutions usually are. Full of popups, trying to get you to buy extra stuff, blah blah. I recommend MSE for lightweight installs/older systems, and Symantec for more powerful/business use systems (not the Norton home crap, the Symantec Endpoint Protection business program). Always back that up with Malwarebytes Anti-Malware, and you should be fine as long as you don't spend your day looking at porn sites.
