After pulling my hair out over the latest update, not, well, updating, I turn to the experts

WU downloads fine, and when I duly start the process, it all looks to install okay, until it gets to the black screen - pretty much all files copy over, but then usually at the first reboot, my HP laptop switches off and doesn't come back on. So I turn it on manually at which point it immediately rolls back to 10420 (or whatever the last build was)

Can't seem to find any error logs so I'm left pulling my hair out - I've removed USB's, I've disabled Norton and still the same problem...

Can anyone help ?