Microsoft support pop up.

I just got home and my father tells me when he opened his browser he got a pop up saying there is malicious software detected and to call this number. We'll, he called.

At this point he had been on the phone for about 45 minutes and had only stopped because he had an appointment. They gave him a number to call back. I canceled what was still downloading to his computer.

What are my chances this was legit (I am assuming it wasn't) ? And what was it doing if it wasn't? What do I need to worry about?