You most definitely have a virus or spyware on your computer. O_O And it sounds like it's a really bad one that your scanner isn't picking up. (Because we literally have ZERO ad scripts or software on the site!)
Which program are you currently using? I tend to have a stash of multiple programs, cause sometimes you'll get that one asshole that your fav program won't pickup, but one of the others will. @.@
I like:
Microsoft Malwarebytes
Spybot Search and Destroy
Avira Anti-Virus
And many people say Avast is really good too.
And if they still don't get it, you might have to google the specific ads that are popping up with a combo of "adname spyware" or "adfilename virus", and you'll usually get a directions of how to track down the spyware file on your computer and delete it.