The lastest intel cpu (6th adn 7th cpu) dont integrated the usb 3.0 driver, so when installing windows 7 on the pc , the mouse and keybord there will be no work.but dont worry, you just need do this steps,you will install win7 successfull.
1. we need a U disk with the win7 system and the tool : windows usb installation Tool, if you don’t have it you can click to download here.
2. runing this software as an administrator.
This step will takes about 20 mins, please wait patiently.
When you finished all you can just close this window, and then USB mouse and keyboard available to use now, pls try to install the win7 system on your pc as regularly you do. Here is link to install windows via USB flash disk: http://support.hystou.com/using-ultraiso-to-make-usb-booting-and-install-win-10/