It's not a virus or any form of illegal software. 

If you installed a dubious program recently or installed one from a company desperate for cash you probably got a new browser extension.

Did you click the "I agree" without reading it? Or did you click it multiple times?

There's an extension going round that underlines words and turns them to ads. 

Look at the extentions/add on's of your browser.

Remove anything you don't remember downloading/using.