Maybe malware doing this to you.
Go into safe mode with networking, do as coastie65 says and download, install, update and run Malwarebytes. Download and run Smitfraudfix while in safe mode with networking. If it has an option to update itself let it. This app should remove any malware related to browsers that have your issue.http://siri.geekstogo.com/SmitfraudFix.php
If this don't kill it get the Kapersky emergency disk from their website, it is free. Boot to it and run it. Now your system is totally inert and the antivirus program should find and delete the problem.http://www.softexia.com/anti-virus/malw ... scue-disk/
Keep us informed as to the outcome.