There is no one program that is virus free. (Norton didn't stop my virus.) Many times it depends on where you go and what you download. All of them have loopholes.
Your best bet, according to my computer programmer son, is a fire wall and many of the anti virus programs out there offer one for more money. I back up on a regular basis, both on dropbox, and my external drive, and use a free one off the web. There are several good ones out there.
Taking care of your data is just as important as taking care of your car or your home. :)