[squirrel1970]

y'know, this is still ongoing and I've yet to find a solution (seriously, either been too ill to do anything or distracted by my friends kittens - little bundles of 'awwwwwwww!') so I think I'm going to have to go this via the 'break the startup'. Let it boot so far then kill the power supply at the plug, then wait a few minutes, start it up again and then take it into safe mode - not sure which safe mode option - with or without networking?
It's not seen any USB devices other than ones plugged into it after it's booted up and installing stuff is a 'no'.
In fact my friend has taken a turn for the worse - it's looking like she may have had a mild stroke after collapsing at home recently, and she's lost a fair amount of ability to talk, so if nothing, the computer is essential now more than ever and I just want to get it running so that she can access it as she always has done, minus the hassle of remembering passwords to access her computer.
I'm going round there this afternoon on four missions, windproof lighter, kitten sexing, seeing how she is and sorting that infernal computer!

I'll take the suggestion of burning an iso to cdr as a final solution. I'm not done yet.
Thanks everyone for help so far, I hadn't forgotten, just got waylaid by other things :-)