I use windows XP. have always preferred to use Firefox as my browser but recently more and more sites fail with the error message Oops sorry cannot find www.google.ad.sgdoubleclick.net. I believe this is the site set up by Google to do with it's internet-based advertising system but as i said it is intervening between me and many, many sites I try to access. I have tried to use the opt out button ostensibly supplied by google but nothing happens. can you advise me?