Norton not only block banners, it blocks text links and strips it all right out of your source code.
In addition to being careful not to name any banners or images with the words ad or banner in the URL also be careful about standard baner sizes. Norton strips IMAGES even if they are NOT BANNERS that are the standard banner sizes.
It also blocks most affiliate banners at the host level.
See this page for examples and more resources including how to block Norton from blocking or altering your page content.
You will also find some in-depth articles I have written for About.com and other sources at the bottom of the page.
NORTON BLOCKS REVENUE
http://www.5staraffiliateprograms.co...s-revenue.html