Re: Virgin Media Limited alerted due to suspicious activity
sounds like malware to me. download malware bytes. the pop ups and the fake windows defender notification will be part of that.
don't phone the number. they will ask to remotely connect to the pc. they go through a set routine of getting event viewer up which lists normal pc errors and say they are viruses, they then type in a text file the cost to sort it out ie hundreds of pounds. they will then either download free software such as cc cleaner and or put a keylogging device on the pc empty the bank account used to pay for the 'service'.
without getting your son into trouble these kind of pop ups are usually the result of using p2p websites/torrents (for illegally downloading films, tv shows, software or music) or even adult material.
apart from malware bytes, do a full virus check using mcafee and on a browser type into google 'housecall trendmircro'. this is a free online virus/malware check. that found something on my pc once when my antivirus didn't.