This is relatively old news, I was saving it for a good day. There are reports that Norton products are blocking AdSense ads from displaying. In this thread, AdWordsRep says;

the problem you're experiencing is almost certainly due to Internet Security software that you've recently installed on your machine (or which has been recently updated).

Norton Internet Security 2004 comes to mind as one that has caused this issue in the past, for others. You should be able to re-set parameters to stop this blocking.

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