Fortunately, if you're running Windows 7 or 8, and you really want to use your free Windows 10 licence key to perform a clean installation rather than upgrade, there is a solution.
It's pretty convoluted, but it works. First, you need to do an in-place upgrade to Windows 10.
You can do this via Windows Update, or if Windows Update isn't playing ball you can use the direct download tool: English/American/German/French/Italian/Dutch. If you use the download tool, make sure you select "Upgrade this PC now." It goes without saying that, before you install any new operating system, you should back up any important data beforehand.
Once the upgrade has completed, make sure you've been activated by Microsoft.
Open up the new Settings app, then head to Update & Security, Activation.
Also note down what version of Windows 10 you have (Home, Pro). Enlarge / This is what it looks like if your copy of Windows 10 is activated. If you haven't yet been activated, you can try forcing it via command prompt: run cmd.exe as an administrator, then type slmgr.vbs /ato. You may need to try a few times, as the activation servers were being hit quite hard. This first step will convert your current Windows 7/8 key into a Windows 10 key.
Now, proceed with a clean installation of Windows 10 (select "Custom: Install Windows only" when prompted.) When it asks you for a product key, click Skip. If it asks you multiple times, just keep hitting Skip.
Once you've installed Windows 10, it should activate normally. Again, you might need to manually slam the servers using slmgr.vbs /ato.
You have a clean installation of Windows 10.
Mod Edit :- The full article can be found here on 'Ars Technica UK' by Sebastian Anthony -
Edited by Chris Cosgrove, 24 November 2015 - 06:59 PM.