http://www.redmondpie.com/download-windows-10-pro-iso-file-without-product-key-from-microsoft/
 
This is especially good for those who have lost their Windows 10 media DVD

Normally, you never had to enter a Windows product key through Microsoft's website to download the ISO. And, each Windows 10 ISO contains Home and Pro.

If I upgraded to Windows 10, could I use this ISO to reinstall on a new HDD?
 
I upgraded to a 250GB SSD boot drive from my small 120gb but so far haven't used it because I didn't have an ISO.

If I did the free upgrade will that key work on a new from scratch install using this ISO?
 
Been thinking about reformatting my system, but I don't want to lose my Windows 10.

Yes, kidcrumb.
 
Yes, fearpoint.  If your computer doesn't use the firmware to store the Windows key, just enter your Windows 7 or 8.x key when prompted.
 
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Even if you haven't upgraded your Windows 7 or 8.x computer yet and use this ISO, it will still upgrade without issue even though Microsoft said the upgrade stopped July 29, 2016.
 
At any time, if you don't want to enter a key when prompted, at the bottom select "I don't have a product key."  You'll have a few days before you will be asked to activate before it disables Windows features, etc.
 
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Clean Install:  Better to boot from the disc/USB - not Reset This PC.
Upgrade:  Run disc/USB within Windows; is more reliable when running it from portable media vs Windows Update.

I don't need one windows 10 was activated on my windows account from my z68 board. you can upgrade your board and windows will activate again fresh as new . you just need a pc on your account with active windows 10 already on it.