Recently, I started getting annoying pop-ups on my screen. I had a tough time removing them, because when I deleted one, another popped up. And there were several. It took a while to get rid of all of them. Shortly thereafter, it happened again. It is quite a distraction, particularly when the ad window blocks a dialogue box of the software you are using.
The culprit was the Google Chrome browser.
It would seem that when you add an extension to this browser, Google slips in a feature that allows ads to be inserted into your computer screen. Lots of them.
There are many remedies. Search for them on your browser, and good luck. I had my tech support service do it as part of my periodic maintenance.