I came across this procedure on another website and tried it on my own machine that had Windows 7 Pro installed in order to verify that it works. This allows you to do a clean installation of Windows 10 using one of the .iso files available from Microsoft, skipping the upgrade process entirely, and ending up with a clean installation of Windows 10 that is activated as soon as you go online.
1.) In order to do this, your current Windows 7/8/8.1 installation must have a valid activation status. No ifs, ands, or buts.
2.) Before proceeding, you must run Windows Update and make sure your system is completely up to date.
3.) Before proceeding, I suggest running sfc /scannow and chkdsk c: in order to verify that there are no corrupted/missing system files and no corruption of the filesystem.
3.) Before proceeding, I suggest creating a backup image of your current system just in case things go wrong, or if you decide that Windows 10 is not really for you and want to go back to using your previous Windows OS.
4.) Go here: https://www.microsoft.com/en-us/software-download/windows10ISO and download the Windows 10 .iso that you will use to create your installation media. It’s up to you to choose the correct version that corresponds to your current Windows 7/8/8.1 installation.
5.) You can either burn the image to a DVD or create a bootable USB flash drive using Rufus.
6.) Here is the first vital step: After you’ve created your installation media, there is a file you must copy from there to your current installation and use it to transfer your activation status between systems. Open your installation media using File Explorer, find the folder “Sources”, then locate the file “gatherosstate.exe”. Copy this file from there to either the Desktop or the Downloads folder of your currently installed system.
Now right click gatherosstate.exe and run it as Administrator. Doing this will generate a new file "GenuineTicket.xml". Save this file to either a SDHC card or a USB flash drive. This is the file that will activate your new installation of Windows 10.
7.) Now use your installation media to install Windows 10. There are two things you must not do:
> DO NOT ENTER an activation key when prompted to do so. This will occur twice during the installation. Just click "Skip this step".
> DO NOT make an internet connection when prompted to do so. Just click "Skip this step".
Once installation has completed, don't do anything. Just reboot the machine.
8.) Here is the second vital step: Open File Explorer and check the box "Show hidden items". Now copy the file GenuineTicket.xml from where you saved it to the folder "GenuineTicket" found in this path:
C:\Program Data\Microsoft\Windows\Clipsvc\GenuineTicket
After doing this, reboot the machine once more.
9.) After rebooting, make your internet connection, then check the activation status. If you did everything right, Windows 10 will be activated.
BTW - I did this purely as an excercise and to verify that it works. Afterwards, I restored Windows 7 Pro from a backup image.