I dropped Norton with 3 months left of my 12 I paid for because the system lag was so bad. With NOTHING else running but XP, it was using around 15 – 30% proc time.
I've had no problems with AVG free, and browsing only with Firefox. My email provider has a pretty hefty spam filter too. (fourbucks.net)
The fact that my internet connection only bogs down on certain maps, and never outside of JO:E (that's noticable anyway) suggests it's not the card. I've streamed NetFlix instant watch with no problems.