Sorry for no screenshot, I typically close my browser windows ASAP when these things pop up. It was an add with this URL (very long) that claimed my system had a Trojan, was booting something, offered a free scan. The usual. There is over a whole page of this URL in my history within the same minute.

Here are the URLs that appear right before this long one:
First - https://adsbg.mediamonetizer.com/13...3t0AxcnJQ%3D%3D
Second - http://clicktrackcarsfast.com/index...,&F=*&G=&H=0.01
Third - http://server1.westmodelworldtodayn...p=99.108.225.92
Fourth - http://critical-alertba10y.stream/g...p=99.108.225.92
Fifth - http://critical-alertba10y.stream/guest/0
Sixth- http://critical-alertba10y.stream/guest/01
From there it adds infinite numbers 0-9 and repeats...

If it pops up again I'll take another screenshot but it would be great if this sort of thing was gone.

Thanks!

That ad was right because those ads are virus, malware and stuff! Only believe warnings that are not on a web page but it's own program pop up in a window entirely separate from every browser and you bought it from a brick and mortar store.

Thanks - @Delphy will contact our advertisers and get that ad source banned.