Whatever you do, don't waste your time with Mcaffee. It's been awhile since I used it but, it's a horrible program. It's incredibly memory intensive and just doesn't work as well as other programs I've used in the past (Norton is quite a bit better if you can believe that).
Personally I would say Avast as well I've been running it for over 3 years now and haven't had to format my system since.
As for my experience ive been in the Tech game for going on 9 years and i must say theres alot of free junk out there but Avast just does its job and shuts the hell up unlike alot of those free ones that bug the hell out of you while your trying to get stuff done like mcaffee.
Its free, Stable, and get the job done. I personally like the Boot scan which it can perform before the Os actually boots.
so yea Avast
Also any you guys feel free to Pm me with any Computer related issues im willing to help.