Heya Vegeta11
I got a Z77 Maximus V Formula, which is very close to yours. So i went into the BIOS to check a few things.
Could you check in the BIOS that:
Under the "BOOT" tab, that the USB Support i set to "Partial Initialization"
Also under the "Advanced" tab that the USB config for "Legacy USB Support" is enabled.
Do you use a Wireless keyboard? If yes. Could you try with a wired one. Since i've had some issues with my Logitech K750 that also made me unable to use my keyboard while installing Windows OS.