... I kicked its ass! The trouble with children is they behave in a childish way. It's hard to train a child to react in one way to a set of circumstances and to react in another way to a set of circumstances that look like the first set, but are in fact different. And so, this is how I came to have a nasty browser highjack installed on my machine.
From time to time, a pop up would appear advertising a certain anti-virus product (how's that for irony eh?). Neither Ad-aware nor Defender had any luck at removing it. In the end I deleted cookies from my cache until I deleted the one that triggered the highjack's initial behaviour. Then I googled the url in the cookie and managed to track down a removal tool.
Sigh, that's a couple of hours of my life I'm just not going to get back.