I can remember a time when OEMs shipped full installation discs with their new hardware.  One of the reasons I purchased Gateway devices was because they always included that install disc which would allow me to do a full clean install anytime I wanted.
When the shipping of those installation discs stopped we then began receiving recovery discs which would restore your system back to its original out of the box state.  Those were initially on physical discs then they were stored on the hard drive and required the user to create set of recovery discs using their own CD/DVDs.
As Windows 8/8.1 arrived on the scene a new system recovery/refresh process was introduced in the OS that built in the process which still accessed a recovery image on the hard drive but it was no longer necessary to make physical discs unless you wanted a backup copy of the recovery image.
Of course that image was built up by the OEMs and meant they could include their additional software within the image so that any reset or recovery made sure their pre-installed software was still on the system.
In Windows 10 that option continues to be available however, if you want to get a clean install which is free of the OEMs extra stuff you must have access to original Windows installation files.
Luckily those ISOs are available for users of Windows 7, Windows 8/8.1 and Windows 10 to download and use to get the plain vanilla install of their OS without all those OEM extras.
With all of these methods you will need a connection to the Internet and storage media such as your local hard drive, a DVD or USB flash drive to create the installation media.
These downloads can be validated and accessed using either a retail or OEM product key. Some OEMs placed stickers on the bottom of their devices with the Windows product key on it however, in recent years they have begun embedding those keys in the system BIOS. You will need to retrieve your product key from the OS before clean installing so you can use it to download the ISO from Microsoft. Retail keys are usually available on the retail box, disc or in an email you received if purchased electronically.
I use a program called Belarc Advisor (Free) to view the product keys for software, including Windows, that is installed on my systems.
The other thing to be aware of is that some OEMs have unique hardware drivers which you may need to download from them directly so your system hardware will work properly after the installation using this media.
Once you are ready to go here are the various ISO download pages:
If you are a student or faculty at a school and you purchased the academic version of Windows you also have access to downloading your installation media for Windows 7, Windows 8.1 and Windows 10 at Microsoft's Download Academic Products page.
For more information on these free downloads you can also visit Microsoft’s Software Download Frequently Asked Questions page.