http://www.tomshardware.com/news/how-to-do-clean-installation-windows-10,36160.html

Information regarding the minimum system requirements for installing Windows 10 can be found on the Microsoft website and are located at the following link: System requirements and important notes.

Note: In addition to the information at the link above, it is recommended that you disable both Secure Boot and Fast Boot in the BIOS/UEFI settings, because some systems may not support booting from USB or Flash drives with them enabled. Also, as a measure of protection against a failed installation, all of the following are highly recommended prior to beginning the clean install process.

1. Check that the software and hardware you mostly use are compatible with Windows 10 and have drivers available, even if your system has already passed the compatibility check. Download and save these drivers to a suitable DVD, flash drive, or external disk so that in the event you need to install them later, you can do so without needing to go find them when it's no longer convenient.
   
2. Create a backup of any important files and folders to an external source or cloud service. A better option would be to additionally or alternatively create an entire system backup image using a utility like Acronis True Image or Macrium Reflect so you can simply restore your previous installation to its exact self if something goes wrong or doesn't work out later on.
   
3. If your only installation media for your current or previous operating system is a factory recovery partition, it might be a good idea to also copy that image to another location. External or secondary drives would be good choices as locations to save these types of images.
   
4. Before you begin the installation process, it might also be a good idea to ensure that your BIOS is set to UEFI or compatibility mode. Legacy modes that may have worked fine with Windows 7 may create installation or hardware issues during or after the installation of Windows 10.
   
5. Temporarily disconnect all other hard drives except the primary drive onto which you'll install Windows. With the power turned off, simply disconnect all other internal drives from the motherboard SATA headers or external drives from their USB ports, except the flash drive or optical drive to be used during the installation process. This step will prevent the possibility of accidentally deleting any files or partitions from those drives when preparing the primary drive for the Windows installation.
   
6. If you have partitions on the hard drive that will be used for the new operating system and are being used for other, non-OS purposes, then back up any information on there to another location because you'll be completely removing all existing partitions from the target drive. Do not forget this step, because any existing partition information on the drive will be eradicated during this process.
   

1. Clicking the link above will take you to the starting page for creating the Windows 10 installation media. Once there, ignore the first box, scroll down the page a bit, and click the "Download tool now" button.

We will assume the use of a flash drive for the purpose of this tutorial, but if you choose ISO file, you will need to burn it to disc afterwards. Make your selection and click next. (You can find information on how to do this here: How to burn Windows 10 ISO image.) At this point, you should have bootable installation media, either on a USB flash drive or DVD optical disk.

Installation

Before you proceed with the installation, it may be necessary to enter the BIOS and set your machine boot order to USB or DVD drive as the first bootable option. Otherwise, it will likely simply attempt to boot using the existing Windows boot partition on your current primary drive, unless this is an installation to a drive that has not previously had any Windows installations on it--or unless you have previously removed all partitions from the primary drive in preparation for installation. Strictly speaking, that is not necessary because you'll do this a little further into the tutorial, as part of the installation process.

1. Start the computer, and after it completes the POST initialization process, it should identify and boot from the attached USB or optical drive containing the installation media and begin the installation. Verify that the language, time/currency, and keyboard input fields are all correct. Click Next.

If you do not have a Windows 10 product activation key, or if you have previously installed and activated Windows 10 and are reinstalling, then click below where it says "I don't have a product key."

Otherwise, enter your Windows product key and then click Next.

(In some cases, especially if you are upgrading, you can use your valid product key from Windows 7 or 8.1 in place of a valid Windows 10 product key. We cannot guarantee this will work in every situation or indefinitely, but currently this seems to still be a valid method of activating Windows 10 product installations. Alternatively, it seems some users have determined through long-term testing that not activating at all seems to not have any effect on the extended usage of Windows 10 other than being constantly nagged to activate.)

If you are doing a new installation due to a change of hardware or in the event you may have a need to do so in the future, the following links may contain information that will be useful to you:

Reactivating Windows 10 after a hardware change

Windows build 1607 and activation after a hardware change

4. Now select the Windows 10 version you wish to install. For most users this will, and is recommended to be, Home version. Educational and other users should select according to the type of license identified on your product packaging or information. Then, click Next.

(Do not delete any partitions showing that are on the USB flash drive you are using to install windows from, if it shows up in the list of partition information. All others may be deleted unless you still have other drives connected.)

If you have not disconnected all secondary drives as was suggested at the beginning of the tutorial, then be very careful to not accidentally delete any partitions from those drives by mistake.

If you still have other drives connected other than the USB or optical drive and the target drive for the Windows installation, we highly recommend stopping now and starting over again from the beginning after shutting down and disconnecting those drives. If there are no other hard drives connected, then proceed with removing all of the existing partitions on the primary hard drive until only unallocated space remains, and then click Next.

(Or so says Microsoft. Some people choose not to use Cortana, disabling all of the relevant features, including a number of privacy related settings, whenever and wherever possible. You can assign settings as you see fit.)

This will make future installations of Windows 10 much easier, especially if you change hardware down the road, because your product key will then be attached to you rather than to your hardware, and you will not have to go through a lengthy phone activation if you make major upgrades to your PC at a later date. For the sake of this tutorial, we chose to enter an email address that of course showed that yes, we do have a valid Windows 10 product activation.

16. Now it will ask you about whether you want Windows to save and sync your information to the Onedrive cloud. We generally recommend not doing this, but some people like Microsoft having copies of all their files and settings, so this is up to you once again.