It's a form of malware that's found its way onto your computer. It can usually be removed by simply downloading, installing and running the free version of Malwarebytes Anti-Malware:
https://www.malwarebytes.org/free/
Some points to note:
1. Take care to deselect the option to install a trial version of the 'PRO' edition of the program ;
2. When running it, select the 'Quick Scan' option ;
3. Don't be surprised when that scan takes over half an hour to run! ;
4. When the scan ends, click to view the full results and then click again to delete everything that's been found.
I've had your problem twice and Malwarebytes has cured it on both occasions.