There could be a number of issues causing this problem, however I suspect the most likely cause of the problem is spyware or adware which has got onto your machine.
You don't specify if you have any anti-virus software, if you have, update and run a full scan.
If you DON'T have antivirus software, go to https://www.avast.com/fr-fr/index and download the free home edition of the software, install and schedule a "boot time" scan (you get an option to do this while installing) - then restart and let Avast run through the scan.
After that's complete, reconnect to the internet and go to https://www.safer-networking.org/products/ and download Spybot Search & Destroy (the download link is about 2/3rds of the way down the page). Install it, check for updates and then run a full scan.
Finally, restart the PC and then see how things have improved - post back here with your results and we'll go into it in more depth if needed.