Thanks for posting.
Yesterday, I visited a new site and experienced this. A screen pretending to be a spyware/virus checker appeared to be doing a real-time scan of the machine and all sorts of threats were getting “x” marks. I happened nearly instantaneoulsy which was suspicious because all my security checkers take a LONG time to scan my machine. Then a fake dialog box popped up pretending to be a MS security alert. I was able to shut the entire browser off, then run MalwareBytes which did find one infected file. Now I need to run Spybot Search & Destroy.
Even after I closed the browser via Windows Task Manager, I was unable to access HTTP addresses, but was able to access HTTPS sites.
I'm now locked down with Fartronics DeepFreeze.
Trolling for graphics to post on FR can be a dangerous practice.
What browser were you using?