Thank you all, your help is much appreciated. I have already carried out a System Restore back to the 11th December and intended to carry out sfc / scannow, but will seek help first as soon as I have time.
Screen / windows are still shaking slightly.
Written on the malicious screen, along with what I already mentioned...............
"Virus alert"
For several reasons - I do not have Behavio(u)r Shield turned on.......................perhaps that would have stopped it?