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From the Adwords forum:
Google Adwords & Norton Internet Security 2004 [webmasterworld.com]
Norton is a mainstream product, and I am willing to bet that many leave their ad blocker set to on, or do not even realize that ads including AdSense and Adwords are being blocked, along with all the animated banners, pop-ups/unders etc. Also, links such as Commission Junction links are also blocked by this program.
This could also explain some of the disappearing AdSense in IE [webmasterworld.com], since this ad blocker is for Internet Explorer only.
I have Internet Security 2003 which is regularly updated. However the ads are visible in that.
My Internet Security 2003 which is regularly updated and set to the lowest default security level shows the ads but clicks do nothing. Now if MS built this into their Longhorn default Internet Connection Firewall settings...
I am still pretty surprised that Norton is blackballing a company such as Google with their own version of Norton Internet Security, but I guess they see an ad as an ad, regardless of what company is serving them.