Sounds like a browser redirect virus, see this advice. (Others available.)

To do the clean-up:

1. Download the latest versions of various security software. My preferred set is the free versions of: Malwarebytes, adwcleaner, ComboFix and HitmanPro. As some security nasties are clever enough to block fix attempts it may be necessary to use a different browser, or to copy the install packages off a memory stick created by different computer.
2. Run these one after the other. With luck they will find and fix all the problems. (Compromised computers typically have several.)
3. Probably won't be necessary because Windows Defender is much better than it used to be, but if the problem persists download another AVM and run a scan with that.
4. It may be necessary to do the above with the computer booted in Safe Mode – Windows 10 howto here. (Safe mode starts the computer in basic mode, in particular it doesn't run most of the user loaded programs & start services liable to be causing grief. Although the computer isn't set-up to meet user needs in safe mode, it can run clean-up software without risk of interference. )

If this is too intimidating, most PC shops will do it for you, for example Curry's charge £45.

To stop it happening in future, try being more aggressive with Ad's by installing Privacy Badger, AdBlock+ and/or uBlock. These browser plug-ins block suspect websites and restrict what can be downloaded. Note they address a species of security & privacy threats that Windows Defender and other anti-virus packages may not.

Dave