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.
1. Check that the software and hardware you mostly use are comptible 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. Make sure to update all necessary drivers and Windows updates prior to beginning the upgrade process, if the installation requires reinstalling your previous OS again first due to a failure in the upgrade process.
(Step 2 does not apply when doing a clean install)
3. Create a backup of any important files and folders to an external source or cloud service. Better yet 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 it's exact self if something goes wrong or doesn't work out later on.
4. 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 as well. External or secondary drives would be good choices as locations to save these types of images.
5. If doing a clean install, 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.
How to do a clean install
One thing to note before moving forward is that the neither of the Reset options built into Windows 10 allows you to remove or reformat drives or partitions, which is highly recommended as part of the clean install process, so it's best to proceed as follows)
Click here to download the Windows 10 installation ISO (Installer image)
Browse down the page and be sure to select the correct bit version for your hardware. Most systems these days are 64bit but there are still some qualified systems and mobile devices running 32bit versions.
Click on your chosen download version and on the resulting dialogue, choose "save as", then save the file to your desktop or preferred downloads folder. Where you save it doesn't really matter as it's a relatively small file meant only to initiate the REAL download.
Find your downloaded file and run the mediacreation tool. Select the "create installation media for another PC" option and click next.
Next, select your language, OS version and bit depth. This is an important step as an incorrect selection here may result in your download being incompatible and necessitate downloading the package over again. Choose "Home" or "Pro" and 32 or 64 bit depending on your hardware. Ignore the "N" versions under "Edition" unless you live in Europe. Click next.
Now on the following screen, under "Choose which media to use", select ISO file, even if you plan to use a flash drive for installation. We'll create the actual media using the Windows 7 DVD/USB download tool later. Click next.
After clicking next, your actual Windows 10 operating system installation ISO will begin downloading to the location you chose. Once the ISO is complete, you may move to the next step below.
Click here to download the Windows 7 DVD/USB download tool and use this tool to create the installation media downloaded in the above step.
(Ignore the fact that it says Windows "7" and "download", as it's really just a Microsoft tool to create bootable media from an existing ISO image and seems to have more success creating bootable USB images than the flash drive option using the mediacreation tool.)
After downloading the Windows 7 DVD/USB download tool, run the utility by clicking on it. The following window will appear.
The following dialogue will open and you should click "browse" and browse to the location where you saved the Windows 10 ISO file earlier.
Select your ISO image by double clicking the ISO file or selecting it and clicking open.
When the browse dialogue closes and returns to the creation tool, click next.
The resulting window will give you the option to create DVD or USB media. Select your preferred method of installation and after making sure there is flash drive or DVD media attached to the system, click the option you wish to use.
Once the installation media creation has completed you are ready to perform a clean install. Proceed to the tutorial linked to below and follow the installation guidelines exactly as outlined. Hopefully, this will result in a clean slate and resolve any issues you were experiencing after upgrading or attempting another recovery procedure that was unsuccessful.
After you complete a clean install, it's probably a good idea to install any specific driver versions you wish to use, especially for graphics card drivers, and then disable automatic driver updating to prevent windows from changing your drivers to undesired versions.
Click here for instructions on turning off automatic driver updating in Windows 10.
Only do this once you are certain all drivers are on the versions you wish to use and everything is working to your satisfaction. Alternatively, you can just leave the automatic driver updating enabled.
*Click here for the Windows 10 clean install step by step installation tutorial.