To clean install on a working system, for Windows 10, just download the ISO from Microsoft
but, how do you establish that the license is valid? And, how do you obtain the license key?
The working, but virus infected, system doesn't have a sticker with licensing information on it. I intend to dual-boot to Linux, but, with various UEFI intricacies, want to know what the options are in a worst-case scenario -- assuming that Windows is totally broken and unrecoverable.
Leaving aside fixing UEFI or other problems. Clean install. See also:
you can download an iso from the MSDN Subscriber Downloads and install a clean Windows 10 with said iso.
How exactly does that process work? Bearing in mind that the working system has a license. (Apparently, applying Windows Updates records the license on Microsoft servers...?)
After Windows 7 that is Windows 8 or later version preinstalled on a laptop, we never need to enter the product key manually. If we reinstall the proper version of Windows (Home, Professional, etc). The installation process reads the product key automatically from the BIOS.
Microsoft has several methods of distributing licenses and entitlements to Windows 10. Depending on the method you obtained your copy of Windows, you may either have a product key, or a digital license which requires no product key. The Digital license is based on your hardware, so during activation it is automatically applied and activated. The following information was obtained here:
Activation helps verify that your copy of Windows is genuine and hasn’t been used on more devices than the Microsoft Software License Terms allow. Depending on how you got your copy of Windows 10, activation will use either a digital license or a 25-character product key.
Digital license (called a digital entitlement in Windows 10 Version 1511) is a method of activation in Windows 10 that doesn't require you to enter a product key.
- You upgraded to Windows 10 for free from an eligible device running a genuine copy of Windows 7 or Windows 8.1. - Digital license
- You bought genuine Windows 10 from the Windows Store and successfully activated Windows 10. - Digital license
- You bought a Windows 10 Pro upgrade from the Windows Store and successfully activated Windows 10. - Digital license
- You’re a Windows Insider and upgraded to the newest Windows 10 Insider Preview build on an eligible device that was running an activated previous version of Windows and Windows 10 Preview. - Digital license
- You bought a copy of Windows 10 from an authorized retailer. - Product key (On a label inside the Windows 10 box. A digital license will be given to your device for Windows 10 based on the valid product key you entered.)
- You bought a digital copy of Windows 10 from an authorized retailer. - Product key (In the confirmation email you received after buying Windows 10 or in a digital locker accessible through the retailer’s website. A digital license will be given to your device for Windows 10 based on the valid product key you entered.)
- You have a Volume Licensing agreement for Windows 10 or MSDN subscription. - Product key (Available through the web portal for your program.)
- You bought a new device running Windows 10. - Product key (Pre–installed on your device, included with the device packaging, or included as a card or on the Certificate of Authenticity (COA) attached to the device.)
One of the most common ways a digital license was obtained was during the free Windows 7/8 upgrade to Windows 10 via the GWX icon. If you obtained it this way, you will not need to enter a product key at all. After the system installs, it will just activate and work.
If you can access the Settings app on your running machine now, you can go to the Update & Security section and click Activation. Here you will either get Activated via Digital Entitlement, or be shown that Windows is simply activated which implies that a Product key was used (either embedded or manually entered).
You don't need to reinstall Windows. Since Windows 8 there is a build in way to reset Windows to the original state, which is the same like doing a clean install.
Start the Settings app, go to System & Security -> Recovery and select "Reset this PC"