we have ads most of the time on the right so if you haven't seen them before I'm guessing something's changed on your computer rather than at this end. I am however hopelessly technically inept so have reported this to tech and he might have a better explanation!
Some threads have adverts on the right (in a block called Sky), while others do not. No idea why it happens to some threads and not others - perhaps it's random based on when a new topic thread is created.
Tech no doubt will be able to give full info, but this is what I have found having played with some settings on FireFox and using Firebug.