We got this once last week and again just now. It opened in it's own browser or covered another site. I had Yahoo mail open as well as Hotmail and a couple other sites that are frequented regularly. I've never gotten this particular alert before the last 10 days.
I could not close or click ok and was only able to close the full screen. I tried that to, one by one, close each browser. There was not time.
I had to manually shut down after a screen shot of the alert.
That's not my IP address: 22.214.171.124
Last week it was 126.96.36.199
Both said call Microsoft immediately.
Upon reboot Trend Micro did a scan and found nothing.