OT: Windows10 issues
LeatherGryphon
Posts: 12,097
In advance of the ending of the free offer of upgrade to Windows10 I've converted one of my machines in the following manner:
The machine was running Vista but I removed the hard drive and stuck it on a shelf and installed a new virgin hard drive.
Then I installed a virgin OEM copy of Windows7-SP1-Home version. (to make me eligible for the free Win10)
Then immediately updated to all the latest Win7 updates. (Over 200 of them. It took nearly 24 hours of waiting/downloading/installing)
Then immediately went to Microsoft and initiated the upgrade to Windows10. (took an hour or two) New system worked fine initially. 
Then installed Norton InternetSecurity and transferred my Norton InternetSecurity license from the the Vista to Win10.
Then made a few other tweaks like adding a second user, rearranging desktop configs, establishing InternetExplorer as the primary browser, etc. I did not install any other applications but somewhere along the line a problem developed and now everytime I log off from either user I get a memory fault.
It might have been Norton or the interaction of Norton and IE but I don't know.
What I want to know is can redo everything from scratch and reinstall the Win7 and is its license still valid or has it been transferred to the Win10? Or do I HAVE to reinstall the Win7 first to get the Win10 download? If I don't have to install the Win7 first can I get a download of Win10 from somewhere and apply my license to it? Or do I have to rollback my Win10 to Win7 and then start over again? Oh, my aching head! 

Comments
Here is an artrticle on your topic.
http://www.howtogeek.com/224342/how-to-clean-install-windows-10/
Thank you. This might work. Head not so achy now.
No problem.
Worked for me - I upgraded from a Win 10 ISO and asked it to remove everything from the old Win 8.1 during the upgrade. After Win 10 had installed I did a new install from ISO overwriting the existing Win 10 (chose "format partition" under installation options) to be sure to get a "clean" Win 10. It said it was already activated when it booted up.
It's worked for me too.
I downloaded the Win10 installer ISO and wrote it to a DVD and used that to re-install over top of the boogered Win10 system. The only trick is that during installation when asked for a key, be sure to say that you DO NOT have a key code because of the way that Win10 activates. If you've activated Win10 on a particular machine once then their database apparently remembers that machine forever and will automatically activate any new installaton of Win10 on that same machine.
I've now gotten everything reinstalled on that machine and am further along than before and have had no weird problems. I don't know what caused my original problem but everything is working OK now.
Glad to hear you got it working. There's often problems with upgraded systems, that's why I always do a clean install from ISO.