When installing Windows 7 on a Z170 motherboard, you will have trouble getting your mouse and keyboard to function through USB if the bios is not on v 1.04 or greater. A BIOS update is available to resolve this which can be downloaded here https://www.evga.com/support/download/

Once you have updated and running BIOS version 1.04 or newer, enter the BIOS (boot options) and set the Windows 7 Installation option to enabled.

Hit F10 to save and restart; now proceed with your installation of windows 7 like normal. Once done with your installation, you will need to enter the BIOS to disable the Windows 7 Installation option.