I switched from Norton to AVG 2 years ago & found much improved performance. Recently however, I upgraded to AVG 8 & noticed my performance has slowed. I'll probably give them some time to optimize 8 before I switch, but it'd be a shame if AVG goes the way of Norton.
PK-Her0, if you're talking about viruses that Norton missed then you had a problem. Norton's problems are being a resource hog, not lack of protection. OTOH there's lots of misconception about the protection various programs provide. Can't comment on Kaspersky, Avast, or NOD32 (yet-I'll probably give them a look in the future) but neither NAV or AVG-AV protects against spyware, and it's questionable how much protection they provide against trojans. Neither spyware nor trojans fall into the traditional category of 'virus'. They're both definitely malware, but not virus.
AVG, by the way, has a specific Anti-Malware version that should protect against those. Not sure about Norton but I'd think their Internet Security version would-but am not surprised if their AV version does not. And that, I think, pretty much answers your question about zombie PC's too. Any product advertised as 'Internet Security' should include a firewall. Whether it's a good firewall or not is another question. A product advertised as 'anti-virus' generally would not include a firewall.