Meant to add that I did a full system scan this morning after having the first scare yesterday and nothing showed up. But one thing I did notice yesterday is that it 'appeared' to be an AVG warning, and as I currently use AVG that worried me, so I shut the PC down there and then. My son reassured me that it was almost certainly a malware thing - confirmed by the scan.
Given what Beavis has said though, it might be back