You must upgrade first from W7 or W8/8.1 to W10, once done you can do a fresh install, no key needed as m$ servers know already your HWID, if it ask for then click on 'Skip'. Done
if you want to clean install upgrade first from W7/8.1 to W10 then backup your activation key with MTK 2.5.3 do clean install, and restore it with MTK works for me -3V66T is W10 generic key
NO need that if already had upgraded the computer from earlier version of Windows! the 3V66T Key is an generic key and will be on all upgraded computers the same around the world!! Microsoft identifies the computer by HWID and if done an new and clean install of Windows 10 again, just SKIP and product key the install ask for abd be auto activated after the first boot at the monet Windows id fully installed.
ok thanks guys, especially for coleoptre2007 and pistal (for classification) but maybe doesn't fully answer my question. beyond issues fresh install which one is the real Key Windows 10 (is 3V66T or another)?
There is no "real Key" anymore..... or if you want to get technical , your HWID is the real key because it's unique to you.
For your already upgraded machine it's the HWID of that machine, NOT an product Key!! That means that, if you need to reinstall Windows 10, and that includes an clean install from Ground Zero, no any product Key ia used or even needed! At the first full boot, while Windows will connect to the Activation Center, your HWID will be read from your Machione, compared with their Database and activated if the HWID is in those Database! The Product Key shown with some apps, is just an generic Product Key like the one you already have!