You can’t make a clean install until you upgrade first time your Windows 7 or Windows 8 copy from Windows.
Here is how to do it:
- Go to this page https://www.microsoft.com/en-us/software-download/windows10
- Download the appropiate Windows version 32-bit or 64-bit using the Microsoft Media Creation Tool.
- Choose upgrade and install the upgrade over your existing activated Windows 7 or Windows 8.
- After you finished the upgrade, go to Settings->About and see if your Windows is activated, if not, then try to check for updates, or do manual activation (without entering any key).
- After Windows 10 is activated then you can make a clean install by downloading again the Windows Media Creation Tool and create a bootable DVD or USB stick.
- Boot the Windows 10 installation from your USB stick or DVD.
- Install Windows 10 normally, because it won’t ask you anymore for a Windows 10 CD KEY.