Avast is a great antivirus. One of the best rated when it comes to catching the "bad stuff." And it being free certainly doesn't hurt. I used to use it before switching to the Webroot, and only then because it would sometimes do updates that weren't really amenable to the Hughesnet plan I had at the time, which was the small daily allowance. Of course, the Gen4 makes that different now, and Avast itself may be different, too.
That you are going to try Avast is a good idea. it may very well be perfect for you. Again, it's rated one of the best, even against the paid types. You can always watch what it's using and if you feel it's too much, change to something else. I would give it a try. And like previously said, do a custom install so you don't end up installing all the extra stuff you don't need.