My wife infected her W8.1 machine with Blockit Ad Remover when she opened an infected yahoo.mail. It is a Chrome extension and can be easily removed. But it comes back daily when she uses her yahoo.mail and open her legitimate emails.
There is no program to uninstall and no program was added recently.
I went to Chrome privacy settings and cleared all the pop-up and plugin options.
It is still coming back and according to my wife is related to her opening her regular emails.
I checked her inbox and they all look OK.