Norton? That's a four letter word to me.
I've been using AVG for years and I'm very happy with it.
I think the only time you should worry about high CPU usage is when your not doing anything. That tells me something else is running that probably shouldn't be.
This may not apply, but it's on my mind, I recommend
Process Explorer. It's basically and enhanced Task Manager. It provides a detailed listing of whats running and allows you to suspend a program withhout shutting it down. I found this very helpful when I was having problems with FS9 stopping everytime I moved my mouse. By suspending programs, temporarily, one by one, I was able to point my finger at Kensington's software.