It sounds like a virus of some sort to me.
Are you able to take a screen capture? Or even a photo taken by your phone or something?
I doubt CCleaner would actually find anything since i'm sure it's not a anti virus/malware scanner. I would try some other anti virus/malware scanners since they all can't find anything. It might be worthwhile running HiJackThis to see what might be running in the background.
Whilst you attempt to resolve this, attempt to use another internet browser. You might have an internet browser specific virus/malware.