TR4; thanks, that was what I was after initially. Unfortunately rather than do a trial and error on which command was causing the problem I decided I'd give Clownfish's remedy a try first because it sounded so logical, and I'm sure it is. But I have 12 network components installed on my master computer and no fewer than 15 on the slave. I have no idea which ones are unnecessary and after playing around with Add/Remove programs I have achieved the remarkable feat of making the kids' computer totally unable to access the internet, a giant leap forward!
Experiments continue at random.....
Any further suggestions are welcome.