"Stop running this script?" -- is this legit?

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.

It's just a popup.

<script language="VBScript">
    Sub myAlert(title, content)
        MsgBox content, 0, title
    End Sub
</script>

<script type="text/javascript">
    myAlert("Web Browser", "Stop running this script?\n\nA script on this page is causing your web browser to run slowly. If it continues to run, your computer might become unresponsive.");
</script>

Either by the code above, which works in Internet Explorer, or an actual slow running script can produce that alert. It is pretty much legit and I believe it rather to be an actual slow running script, because who in his right mind would reproduce that box using VBScript?

It's not scam or malware.

They don't ask such stupid questions, clicking on Yes or No won't suddenly install something on your PC.

At best, it is just a joke that you can safely ignore...

What does the Internet Explorer warning look like?

Execute the following in your address bar and wait for a minute or so: javascript:while(true);

Somebody wrote a bad javascript, with an infinite loop or other CPU-intensive process. Those are usually easy to detect during development, but in a more complex scripts oopsies like this happen sometimes. I've done my share of those... annoying, but harmless.