a computer the kids use so, personally, I think they may have downloaded something that triggered this
My solution to this was to unplug the network cable from the kid's computer "until I got round to sorting it out" ... and this "until" doubled on each occurrence.
It took me a week to get round to it the first time, and a fortnight the second time. They know that if they choose to download and install and run any viruses again it'll be a month next time.
And that seems to have done the trick. No viruses on any child's computer for several years now.