I get this popup:
It looks rather suspicious since it refers to "web browser" rather than a specific product, and doesn't offer any other details or identifying information.
I've tried to figure out what dastardly deed it could be doing and all I can think of is that it could be somehow superimposed over a Windows security authorization request (though I have no idea how one would do that).
Added: I got this in the late phases of a reboot after a crash (some Microsoft mystery). To my recollection, no browser was active when the system crashed, and I don't have anything set to autostart on boot.
I'll also add that I checked Process Explorer while the message was up and saw no sign of Mozilla (my usual browser). I forget what (if any) task bar name may have been displayed, but it was not useful.
I tried X-ing the box, but that was ignored, so I clicked "No" once. The window went away and then popped up about an inch to the left and higher up. Clicked "No" again and it went away.
I have seen a similar screen when using a browser on a bad web page, but to my recollection it always identified the browser, and the web page/script was obvious from the context.