How to Perform a Clean Install of Windows 10 After Upgrading

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Aside from that fact that ANY time you move to a new operating system, or even have had the same OS installation on your machine for a long period of time, doing a clean install is a highly recommended procedure to eliminate accumulated garbage, random registry entries and file system corruptions, I think the bottom line here is that there have been many users with less than satisfactory experiences after upgrading to Windows 10 from Windows 7 or Windows 8/8.1.

And just to clarify, doing a clean install of Windows 7 or 8, then installing Windows 10 OVER that installation, is an upgrade, not a clean install, and will not have the same results. There are apparently, a number of issues that in fact are a result of the upgrade and cannot be eliminated without removing the older operating system from the equation entirely.

Please also note that Windows 10 DOES now accept Windows 7 and 8 product keys if this is the first time you are installing Windows 10, so going through the upgrade process just to THEN turn around and do a clean install is no longer necessary, but that method is retained here just to offer multiple options.

The reasons are many and run the gamut, from having a long installation history with many prior old application installations/uninstallations, prior existing issues related to previous operating system or driver installations, unsupported or seemingly unsupported hardware, drivers that are unstable due to multiple installation instances without fully removing the related registry entries and supporting files or even just a nasty case of "bad luck syndrome".

OR, you may have already tried to revert to your previous installation using the rollback feature or tried to do a refresh, reset or restore and ended up with a mess.

"Why" you're having issues after installing Windows 10 probably doesn't matter near as much as the fact that you ARE. In some cases you may simply have unsupported hardware that has slipped through the compatibility verification process or was considered non-essential by Windows but in reality is essential to YOU. Regardless of the specifics, one thing is certain, windows isn't working right and you NEED it to.

For a lot of users with these problems and even just as a matter of standard procedure for others with no glaring issues at all, a clean install has been an effective solution. If you have already completed an upgrade and Windows 10 was activated, you can move forward with a clean install and should not have any activation problems. If you have not completed an upgrade, do not perform a clean install until an upgrade has been completed and activated unless you have purchased the stand alone operating system and do not require doing an upgrade.
I can't tell you that a clean install will resolve every issue out there, because there are clearly situations where it won't. If there simply are no compatible windows 10 drivers, there is faulty or failing hardware or the system just doesn't meet the necessary requirements but an upgrade was forced anyhow, a clean install may not resolve the problem. If your system is compatible however, a clean install will resolve the problem in the vast majority of cases and at the very least you'll know you're working on a clean slate, which eliminates a lot of speculation. Here's how.

The following has been compiled from various sources including service and test installations, recommendations from other members and the Microsoft website.
(Special recognition to member Samat for his preinstallation recommendations and moderator Someone Somewhere for pointing out the use of the Windows 7 USB/DVD download tool with the Win10 ISO.)