I use a AVG free and Bitdefender combo.
This since a while back a new worm came out, which took a couple days to be recognised by AVG. Bitdefender Internet Security features as well a Spam filter and Firewall.
Normally every program has weaknesses; that's why I use 2. Anyway, AVG doesn't have any impact on th performance of the PC, so its no penalty.
Avoid Norton, since that one is honestly overrated.
Good luck on your choice!