How to install Windows 10 on an old computer after many failed attempts PART 2 (more detail here)
PREPARATION CONTINUED -
7. In file explorer, C: drive, look in Program folders, User Application Data and AppData to check if there are any folders or files left over associated with the programs you've uninstalled. I found quite a few. Check programs you've kept >Control Panel>Programs and Features. Delete any leftovers in File Explorer. If necessary do this as Administrator.
8. Deep clean your registry. Restart.
9. Update your drivers. Restart.
10. Make sure you have the most up-to-date BIOS (for my Dell=A12)
11. Unplug all devices, USB devices, speakers, printers etc. leaving connected only a wired keyboard and wired mouse, and modem/router.
12. Uninstall your Antivirus and Firewall programs. (Yes it's OK. You aren't doing anything much online at this stage.)
13. Clean your registry again. Restart
14. Clean boot. Restart
INSTALLATION PROPER -
All of the above are the preparation steps. This is the Upgrade step. Insert your .ISO USB stick in a USB socket on your computer (NOT a USB hub.) In File Explorer look for the setup.exe file. RIGHT CLICK it and Run as Administrator.
You will soon see a blue rectangle with progress stages. At the appropriate stage, choose Check for updates, Accept licence agreement, Keep Personal Files and Programs/Apps. This stage loads files onto your computer.
Update progress is shown as a percentage. When these stages are finished, there will be an automatic restart and then the Upgrade Proper will begin, shown on screen with a large circle and a progress percentage in the middle. There are three stages to the upgrade with auto-restarts between each. Remove the USB stick before the first of these restarts.
If all goes well it'll go to completion (100%) and you'll see a screen 'Welcome to Windows 10' etc.
Wait for some housekeeping to be done and you should eventually see your desktop with your shortcuts on it. If the screen's black (mine was) you can put a picture there by clicking Start (BOTTOM LEFT), Settings, Personalisation, Background and click on a picture.