Before doing anything else, download, install and run the free version of Malwarebytes Anti-Malware:
https://www.malwarebytes.org/free/
That's fixed the underlining problem for me (twice) and has also done so for many others here on AB. (There has been one case where more complex measures were needed but let's just try the simple stuff first). That program also gets rid of most pop-up problems as well.
Some points to note:
When installing the software, take care to deselect the option to accept a trial of the (commercial) 'PRO' version ;
When running it, use the 'Quick scan' option (but don't be surprised if it takes over half an hour to run) ;
When the scan finishes, click to view the results and then use the option to delete everything that has been found.