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If you have a version of Norton installed that includes ad blocking, but you had disabled it, SP2 may well have re-enabled it for you. This happened to a friend of mine.
Norton is hateful, I lose much money due its stupid "anti-ads by default"! :-(
I dont think so. I use urchin to monitor my traffic and I notice that there is a difference of 10 - 12 % in traffic reported by adsense and urchin. There must be a lot of surfers using Norton Internet security and similar software which blocks adsense.