What follows was correct in January 2018, when it was written. I anticipate it should remain correct into the foreseeable future, but depending on what Microsoft might do with the Media Creation Tool or changes made by Rufus certain aspects of these instructions could become obsolete.
Doing a Completely Clean Install of Windows 10
1. Download the Windows 10 Media Creation Tool (MCT). Run it and choose the option to download media for another computer. The MCT defaults the language and architecture to match that of the machine running the tool. If you need to change either one for your target machine then uncheck the checkbox that controls this and tweak the dropdown for language and/or architecture [32-bit or 64-bit] appropriately. When you arrive at the dialog that prompts for what you want to download, elect to download the ISO file rather than to create a USB flash drive. I say this because if the flash drive creation fails for any reason you lose access to the ISO that was downloaded behind the scenes to create it. If you download the ISO file then if the creation of the bootable media fails you have it right there to try again.
2. Use Rufus (or a similar utility of your choosing) to burn the ISO to a flash drive to create bootable media. The steps that follow presume you’re using Rufus. Attach your flash drive and run Rufus. In the first dropdown box:
A) If you know your machine is newer hardware that uses UEFI as opposed to BIOS, select GPT partition scheme for UEFI. If the machine originally came with Windows 8 or Windows 10 this is definitely the choice to make.
B) If you know your machine is older and uses BIOS, or are uncertain as to whether you have BIOS or UEFI, select MBR partition scheme for UEFI or BIOS. Virtually all machines that came with Windows 7 (and earlier Windows) were produced with BIOS and use MBR partitioning.
Leave the other checkboxes checked or unchecked with their defaults.
3. Find the Create a bootable disc using checkbox and make sure that the option ISO image is selected from the dropdown next to it. Just beyond that dropdown is the Click to select an image . . . button. Activate it and you will be presented with the standard Windows browse dialog that lets you locate and select your ISO file from wherever you saved it.
4. Activate the Start button. The ISO image will now be burned to the flash drive as bootable.
5. Boot your system from the USB flash drive. [Since I don’t know what UEFI or BIOS you’re using it’s up to you to find out how to change the boot device order on your machine to put the USB drive first in the boot order]. When you get to the screen that asks for language, press Shift + F10 (or the Context Menu key if your keyboard has it) to open a Command Prompt or PowerShell (it doesn't matter which comes up). Type the following commands:
diskpart (you will most likely get a UAC dialog, to which you should, of course, respond "Yes")
select disk X Where X is the disk number on which you wish to install Windows 10. This is usually 0. Make sure you have the correct number as the next step will
wipe the disk of all partitions.
clean Purges the disk of all existing partitions
convert gpt Initializes the disk as GPT required for booting on a UEFI motherboard. If you get an error ignore it. If and only if you know you have a legacy system that
uses BIOS, use convert mbr instead of convert gpt.
exit (to close diskpart)
exit (to close Command Prompt/PowerShell)
The install will now continue; answer any prompts appropriately. When you get to the screen asking on which disk you wish install Windows, activate the Next button. The Windows 10 installer will automatically partition and format the drive using the format you previously specified in the convert command above.
Edited by britechguy, 15 January 2018 - 10:14 AM.
Made minor changes regarding start of diskpart and need for two exit commands