Although this is an old thread, this is smthn that'll happen as long as we use our cellular devices. Hate to say this but from the answers i did read.... those are all bandaids, so to speak. Are as my instructions honesty just a bigger, thicker bandaid. Moving along now -
Obviously you were in your browser when you got that pop up, so you need to clear everything out of your browser (internet).
In your settings menu, tap applications. Find your browser (or internet) and clear it out, I mean completely. Last step: Reboot/ restart your device.
I will say this once you get that pop up... you know, "you've won this you won that" the "choose your free gift now" "the five top winners" lol complete with comments of contest participants... etc etc etc etc, then most likely you will get that pop up again at some point in time. As far as i know though, it won't destroy your data or your device -AS LONG AS you address the matter. Personally, I've conditioned myself to treating it as smthn that just goes hand in hand with my android devices. Kinda like my Swype feature. I took screenshots the last time it happened to me, approx a year ago and I researched it. Lol. Even down to the comments of the (not real) people.
Don't freak out abt it though. It'll all work out.
Good luck.