Q: Can I clean install (i.e. boot from media and install) Windows 10 on my Windows 7 or Windows 8.x device and still take advantage of the free upgrade offer?
A: No. Clean installs of Windows 10 on a Windows 7 or Windows 8.x device via booting from media are not considered “upgrades”, so the free upgrade offer will not apply. For your Windows 10 installation to be considered an upgrade, you must start installation of Windows 10 while booted in Windows 7 or Windows 8.x
If you attempt to clean install Windows 10 on a device that hasn’t taken advantage of the free upgrade offer and successfully activated Windows 10 online before, you will be prompted to enter a Windows 10 product key to continue installing Windows 10.