I know this HOWTO is for a different device, but Windows not detecting a phone in fastboot mode is a rather frequent problem and the solution depends on Windows and not so phone model. So the steps described there should help you, too.
I know this HOWTO is for a different device, but Windows not detecting a phone in fastboot mode is a rather frequent problem and the solution depends on Windows and not so phone model. So the steps described there should help you, too.