Gah - you were correct. It was that stupid SmartScreen thing - the thing that usually says the source is unverified when you try to install. For some reason it didn't even pop that up this time.
Thanks!
No logs really to give on this one...
Downloaded the new installer, and it won't launch. Just sits there with the rotating pointer and crashed Windows Explorer. Never had any install problems prior with the installer.
Running Windows 10 x64.