Norton and games dont mix, they never did.
Norton takes also your freedom of choice, takes control of your PC.
If you think by using silent mode, etc..thats what Norton wants you to believe.
Your game will be always 10% or more slower because of Norton.

But the decision is yours, check these experiences from serious gamers:

http://www.eurogamer.net/forum_thread_posts.php?thread_id=72231