I have been using Free Avast! for several years on about 8 different computers that I deal with. It works great, the only hang up is that once a year you have to take a moment to tell them you still want to use the Free version.
I also STRONGLY recommend to the people that I do computer work to use Malwarebytes Registered version. It runs quietly in the back ground and works well with Avast and block a bunch of junk that tries to sneak in. I keep a copy of SuperAntiSpyware on an emergency thumbdrive and also installed but not running all the time. If I think I have stuck my browser into something I shouldnt have I can use it and Malwarebytes to run a sweep for adware, virus or bad cookies.
I watch Newegg sales and try to keep a couple copies of Malwarebytes in the cabinet. You can get them for under 15 bucks when on sale and its a lifetime registration, so no annual mystery withdrawals from the bank
When one of the family brings me a computer to fix, I just throw a copy on it for them
I keep a copy of CCleaner from Piriform.com installed and use it to clean up my browser cookies, Temp files, and registry every month or two.