From what I have read, you do not need the Windows 10 key when doing a clean install as long as WIndows 10 was upgraded from 7/8.1. The computer's hardware ID's are stored on Microsoft's servers.
@Union Thug you beat me to it. Sorry, didn't see your post.
In previous Windows releases, when you installed an upgrade version of Windows, if you wanted to reinstall the upgrade version, you had to first reinstall the qualifying version from which you upgraded, reactivate it, then upgrade again and reactivate again.
With Windows 10, this is no longer the case. Once you have upgraded to Windows 10 and activate, you can simply create a bootable copy on DVD or USB and reinstall just Windows 10 without the need to reinstall the qualifying version and it will reactivate automatically.
"Windows 10 handles keys differently. When you upgrade to Windows 10 via Windows Update from Windows 8.1 or Windows 7, the process registers your computers hardware, and qualifying OS with a Microsoft Product Activation Server. The device is then considered a Windows 10 device.
Subsequently anytime you re-install (or clean install) the installation checks with those servers, finds the unique installation ID and produces the validation"
Edited by JohnC_21, 02 August 2015 - 09:33 AM.