Hello, my friend
I'm sorry, but right now there is probably no way, how to make fastboot work on Windows 8.1. Trust me, I've tried to find a solution for many days, but at the end, I had to use borrowed netbook with Windows 7 installed on it. The problem is, that 8.1 always connects fastboot devices as "Unknown USB's". For those you simply can't install these drivers, even if you try using Safe mode, or if you start system with allowance of unsigned drivers. Basicly what's happening is, that windows thinks, you want to reinstall USB driver, not fastboot driver.
I was thinking about the possibility of using older virtualized system or Linux on which it should work (though you won't find many guides). Keep on trying and don't give up
You'll manage to unlock it eventually. The question remains, on which system it would be.