After talking with Spectrum, it appears that the warning was legit, but no one knew how or why the "banner ad" was displayed. I'm guessing that Ccleaner has an information-sharing agreement with Spectrum or f-secure, who provides their botnet scanner -- I used the same email address signing up for this forum that I use for Spectrum. It also seems that Spectrum attempted to contact me via that email address, but I never got it, because it's a Hotmail address, and it seems that Hotmail sometimes filters out mail from certain sources without even sending it to my junk mail folder. So Spectrum used this creative means to get my attention...