Thanks for the advice here. I've recently had exactly the same issue with mgid pop-up ads.
I followed the instructions on this thread and the problem was resolved. However, I've noticed it's back again. But I haven't used or even clicked on any mgid page (as far as i'm aware). I even looked through my cookies to see if i'd stumbled upon it by mistake, but i hadn't.....so maybe mgid has hidden a cookie somewhere else on my machine. But surely a cookie cannot open a pop up by itself? I'm worried that maybe it has installed some kind of malware!
Any advice or suggestions?