64-bit versions of Windows 10 and 8 include a “driver signature enforcement” feature. They’ll only load drivers that have been signed by Microsoft. To install less-than-official drivers, old unsigned drivers, or drivers you’re developing yourself, you’ll need to disable driver signature enforcement.
With Windows 10’s Anniversary Update, Microsoft tightened the screws even further. But you can avoid the more restrictive driver-signing requirements by disabling Secure Boot.