Well it was a clean install on a new HDD, but it was actually 8.1 and it asked right away if I wanted to upgrade to 10 for free before 8.1 even launched. My bad on that post, it was initially a W7 machine, the free upgrade to 10 destroyed it and I had a copy of 8.1 Pro so I used that instead of the OEM key. It happened during the 'checking online for updates' process of the install.