Oh dear, the power supply has blown on the 2006 PC, so I've called it a day on that one - and being an IDE motherboard, not worth spending any money on it.

I transferred its HDD containing 32bit Windows 10 to another PC which booted up OK. I used the Media Creation Tool to download a 64bit version of Windows 10 to a USB drive from which installation was started. All was fine for a while but after a reboot it stuck on a black screen with flashing cursor. Did the same on 2 further attempts, but oddly after putting a Windows 7 Enterprise disc in the dvd drive and rebooting, it gave a blue screen with "just a minute" and after a while showed set up questions - home and dry!!

It's now doing updates and will be a clean spare drive.