There are two different fastboots. They might be using different USB VID/PIDs.
That means that you might have to load Windows drivers two different times.
Code:
$ adb reboot fastboot
gets you the user mode fastboot that says "fastbootd" at the top.
Code:
$ adb reboot bootloader
gets you the boring, normal fastboot without any fancy screen.
That's the one that you are probably missing drivers for.
Go to Device Manager and look for something with an exclamation mark and without a driver.
You can also use
UsbView.exe to see what you have when you go to the real bootloader.