Get something more effective than Windows Defender. It can be handy to check for changes to startup entries, but little else. As an experiment, close all browsers and go the start orb, type in the search area
and press return
(replace the dash with a dot of course, you have to type the whole address yourself and not just click the link)
Do you get a browser showing just the google page or google and all your usual startup links?
If the former is the case, and I suspect it will be, some kind of spyware is generating popups and opens the browser to a single page without your input.