I tried using my win7 key from Technet as I had heard that should work without having to upgrade. So I did a plainjane install.
The key doesn't work, but will allow you to keep it installed on the machine for 30 days. I messed around with it on a brand new install on my office machine so it wasn't an upgrade from W7 to W10. Wasn't impressed reformatted/reinstalled W7 with the Technet key I had and it worked. I don't think I'll upgrade either machine until I'm forced to.
I tried the same thing, downloaded the ISOs from MS so I could do clean loads instead of upgrade. After that didn't work (said wrong key) I googled around and found the workaround.
Turns out that the way they are able to let everyone upgrade from anything Win7 upward is that they issue and register a new product key for Win10 during the "official" upgrade, assigned to your hardware. You can do a clean load after that with the Win7 key. Extra steps that first time, but no crapware.
Oops! The "new" product key shows up in your computer properties after the upgrade and can be used for a clean reinstall of 10. The buried key is what the upgrade uses to validate your old OS, so you shouldn't need that one.[/edit]http://www.howtogeek.com/224342/how-to-clean-install-windows-10/
Another thing I learned (by failing first) is that it will not accept an OEM key from Windows8 for a clear install of Windows10.. But you can paste the code below into a cmd window to get your "original" key (Win8 only) that doesn't have letters in it. Do this BEFORE the upgrade, because the command does not work in Win10.
WMIC Path SoftwareLicensingService Get OA3xOriginalProductKeyhttp://www.eightforums.com/installation-setup/38486-need-help-formatted-genuine-windows-8-lenovo.html#post330277
I have a refurb HP laptop (AMD E1-2100 with 4GB, came with 8.1) that I got with a promo code at Fry's for $150 that will someday in the near future replace a severely limping old Dell netbook as my wx and media server. Good thing you don't need any kind of power for those tasks, because Win10 has already brought it to it's knees, compared to Win8. It takes up to 10-seconds to load a jpeg from the desktop, and have lots of patience waiting for webpages to open with Edge (already put Chrome on it out of impatience). Desktop search keystrokes remind me of watching someone type across a 300-baud modem from the lag.