That is the iNews but it shouldn't continually pop-up it only appeared once in mine. It was 100 million not 1 billion though.
I think that this is the link though,
http://www.avast.com/eng/100m.html. However as an existing user you couldn't be the 100,000,000th user, of course you could try registering again with different details, but that kind of defeats the object of the 100,000,000th user milestone. Not to mention timing, as you don't know what the number is at present, the rate people are registering, etc. to jump at the right point to hit it on the button.
