Windows 10 has been a reasonable success since its release, but if you need, or want, to install an older version of Windows (software compatibility) - or perhaps like me you prefer Linux, it is practical to uninstall Windows 10 and install whichever OS you prefer. Unfortunately, there’s no automatic system for older versions of Windows — you will need to back up your files, find some installation media, and find your OS software key to get back to your “old” computer and validate it.
Though the guide below is written with rolling back to Windows 7 in mind, the basic steps work for any Windows operating system going as far back as Windows XP, though we do realize that if your going that far back, Microsoft ended support for it. Also, be aware that new computers, may contain new hardware components that weren’t manufactured when older Windows versions like Windows 7 and Windows Vista were being sold. That being the case, the manufacturer of your computer and/or the OEM supplier that created the parts may - or may not have working drivers available for the older version of Windows.
What you will need
Before beginning anything else, back up your important computer files to a separate location. An external drive or a cloud storage service is fine, so long as it’s physically disconnected from the Windows computer you’ll be working on. Windows can’t preserve programs or settings when moving to an older version (only a newer one), so you will to get installation media for any crucial programs, especially those that require activation codes or serial numbers.It is a good idea to download essential device drivers for your computer’s various components before you begin the installation process. This will make it easier to get everything up and running if Windows doesn’t automatically select the appropriate drivers. Ethernet, Wi-Fi, and graphics drivers in particular can make the setup process much easier. Store these drivers on a USB thumb drive for easy access once Windows is finished installing.
If you’re installing Windows 7 (or older), you’ll need the original disc (and possibly an external CD or DVD burner, if your newer laptop doesn’t have a disc drive). Alternatively, you can create a bootable USB drive with a Windows ISO.
You’ll also need the Windows product key for whichever version you chose — this is the 25-digit code that came with your Windows retail box or purchase receipt from Microsoft, or is located somewhere on your computer’s case.
A note on Windows 7
While it may be starting to show its age, Windows 7 is still of interest to power users looking to replicate or fix old software, or people who are just stuck because of older application software. Windows 7 can’t upgrade, install, or keep files when installed on a system already running Windows 10, so a fresh install is the only way to achieve such a goal.
You can reinstall Windows from scratch using the product key that came with your PC, but you’ll have to find installation media yourself. Microsoft offers free ISO files for downloading; you just have to know where to look.
Download the Windows 7 SP1 ISO Directly From Microsoft’s Website
Microsoft makes the Windows 7 SP1 ISO available for direct download through their site. The only catch is that you’ll need a valid product key in order to download the file–and OEM keys (like the one that came on a sticker under your laptop) won’t work. If that’s you, proceed to the next section.If you do have a valid retail key, head to the Windows 7 download page, enter your product key, and click “Verify” to start the download process.
After your product key is verified, select the product language you want to download and then click “Confirm.”
Next, choose whether you want the 32-bit or 64-bit version of Windows 7. When you click whichever version you want, the download will begin. Note that download links generated by the site are only valid for 24 hours. Of course, you could always come back and walk through the verification and selection process again to generate new links.
After downloading the ISO file, you can burn it to a DVD by right-clicking it in Windows Explorer and selecting “Burn disc image” to burn it to a disc. If you want to install Windows 7 from a USB drive, the best way is to use the Windows 7 USB/DVD Download Tool to put that ISO file onto a USB drive.
The downloaded ISO you’ll get from Microsoft includes Windows 7 with Service Pack 1. When you install Windows 7, you can avoid the hassle of downloading and installing the hundreds of updates that came out after SP1 by installing the Windows 7 SP1 Convenience Rollup. Even better, why not take a little extra time and slipstream the Convenience Rollup right into your Windows 7 ISO? That way, whenever you install Windows 7 in the future, you’ll have one ISO with all the updates (at least up through May 2016) already included.
Download Any Windows or Office ISO Using a Free Third-Party Tool
Microsoft used to make all these ISOs available through a site called Digital River, but it doesn’t anymore. Instead, they’re stored on its TechBench site. The ISOs can be hard to find, though, and for versions of Windows other than the most current, the site tries really hard to push you into using the Media Creation Tool instead. Enter the Microsoft Windows and Office ISO Download Tool. This free utility provides a simple interface that lets you select the version of Windows you want, then downloads an ISO for that version straight from Microsoft’s download servers. This includes various builds of the Windows 10 Insider Preview. You can also use the tool to download ISOs for certain versions of Microsoft Office (Office 2007, Office 2010, Office 2013, Office 2016, and Office for Mac).First, head over to HeiDoc.net and grab the Microsoft Windows and Office ISO Download Tool. It’s free and it’s a portable tool, so there’s no installation. Just launch the executable file. In the main window, choose the version of Windows or Office you’d like to download.
In the past, Microsoft provided disk images for many of their products through their subcontractor "Digital River". These downloads were pulled in early 2014. Afterwards, Microsoft made a limited selection of downloads available on their TechBench site. The above tool accesses that TechBench site, and unlocks a large number of hidden download files on it.