If you tried twenty different ones, each would find something the others missed. I put Microsoft Security Essentials on all machines. Nothing is completely safe. If someone wants to get into a computer, there's very little you can do. The best defence is you. Most problems are from people inviting mal-ware on to their computers or when installing "free" software, not doing a custom install. This will avoid browser hijacks and being locked out of email accounts etc.
A lot of people here have laptops, some of them quite old. While Norton has been around for eons (way before internet) it is a big beast and really hurts older, slower machines. Security Essentials has what's called a small footprint, and is quite gentle on older machines. And it's "FREE". If you do install it though, watch out for tick boxes, about joining schemes etc.
Having said all that, there is some very good free software about that give no problems at all.