I'm having the same exact problem. I hope someone replies with a fix. This just started a few days ago.
I have my pop up enabled and don't usually get pop ups but suddenly I keep getting the same pop up - cdn.fastclick.net - My options are SAVE - RUN - CANCEL . I click cancel and it pops up again - I click cancel and so on. I deleted all my temporary files and cookies. I did a complete malware scan and a complete AVG scan. How do I get rid of this pop up and what is it?