Malware adverts. STW is apparently known for them sometimes if you don’t have a ‘P’ 😉
They haven’t infected anything but they are irritating and difficult to stop at times. They work on flaws in the majority of browsers in that they let pages overlay on the whole screen and use scripts to stop you clicking close etc. Worse is modern browsers try to be helpful and restart the last page you were on if the browser was shut down by force, e.g. using Task Manager to kill it or rebooting or powering off the computer.
Edge browser at least now will ask you if you want to resume the tabs. Chrome possibly also.
Just ctrl+alt+delete, kill the browser process, then start it again, answer no to resuming tabs.
I wouldn’t resort to installing random anti-virus software. Half of them are malware themselves (in my opinion even legitimate big name ones, they are more damaging to your computer than the decent built in anti-virus in Windows 10).
If you want to double check, try Malwarebytes (free version).
