How to install Nexus 6 / 5X / 6P Drivers in Windows 7 properly where fastboot can see it?
ADB devices was detecting the phone, but fastboot devices wasn't. I had to do the following to get it to work on Win 7 64bit:


Reboot Win7 and Press F8 on boot screen and select: Disable Driver Signing 
Boot phone into bootloader
Device Manager you should see an Unknown Device popup
Manually install the driver by selecting: android > android adb (screenshot)
Then goto command line: fastboot devices 


Let me know if it worked or not. If it did, all I ask is thumbs up this post! I spent a good 3 hours to figure it out. No where on the net has a solution that I've found.