It's a little messy for Windows 10 testers, but it's the guaranteed way to ensure you don't lose activation.

I tried clean installing and I lost activation, but restoring to a Windows 8.1 install to upgrade to Windows 10 and then format and clean install worked. You can try going for a clean install using the ISO, but it's not guaranteed to work unless you've officially upgraded that machine to the final version of Windows 10 from a Windows 7 or Windows 8.1 install.

This may have been overwritten by Windows 10, so if you're not able to restore then contact your PC maker. You can restore your PC using the DVD your PC maker supplied with your PC, or attempting the reset PC process on your machine. To be 100 percent sure you'll retain your product key you'll need to restore your PC back to Windows 8.1 or Windows 7 and then perform the upgrade process detailed above. First things first, backup everything you want to keep on your PC. If you've been testing Windows 10 and now you want to clean install it on your machine, things get a little tricky. Once you've installed Windows 10 it should automatically activate, but if it doesn't then you can run slmgr.vbs /ato from a command prompt to initiate activation. If you prefer a clean install, you can select Create installation media and then provide a valid Windows 7 or 8 key while installing Windows 10. Ignore any product key prompts along the way by selecting the skip option. There will be a variety of setup prompts, but ensure you choose the custom installation to format your existing partition. Once your USB key or DVD is ready, reboot and use the BIOS options on your PC to boot from the USB or DVD. When you pick the version of Windows 10 ensure that it matches the version you noted down after the upgrade. You'll need a USB key that's at least 3GB in size or the ability to burn an ISO file to a DVD later. Write down the exact copy of Windows 10 you have installed, and then download the Windows 10 installer again on your machine and this time select the second option: create installation media for another PC. Check that your PC is activated and check the copy of Windows 10 you have installed from Settings > Update & Security> Activation.

Once you're on Windows 10, ensure that you login with your Microsoft Account you intend to use.

Clean install windows 10 from windows 7