Beware of malicious pop-ups

That particular one is most often associated with malware/adware running on your computer. The one Alex is mentioning is introduced using malicious ads appearing on web sites.

Regardless, don't be complacent thinking you're 100% protected by your primary antivirus and use a second line of defense for the ones your primary doesn't catch.

One of the better options is to simply download the free version of MalwareBytes from https://www.malwarebytes.com/ and periodically run a manual scan with it. It's often suggested to do that by running your operating system in safe mode with networking due to the level of sophistication of some malware.

I recently helped an individual using a very reputable antivirus, on running MalwareBytes it detected some 500+ threats that her primary antivirus had missed.    
       

Thanks for the good info, I installed the Malwarebytes and it found numerous malware type items including 2 buried in the registry which Essentials found but couldn't remove them and only quarantined them. One of my sons is an MCSE  and the chief nerd in the IT section of a Major Co. I guess I should give him a call. but I'm a little stubborn......

In the case of the individual mentioned above, her issue was Windows Explorer (the file manager) downloading something to the tune of 1GB a day (probably all the other malware) and blowing through her data cap.  Only caught it using Glasswire where the culprit was made clear and a quick download of MalwareBytes and scan identified it as well as the others removing them with a single click.

Her experience should serve as fair warning to all who state: no way I'm using that much data... sometimes no way, becomes big way (but not always YMMV).