scratch everything I said
you can't it needs to be 10 =/ (unless your board has XHCI drivers in the WU repo which most do NOT)
http://wccftech.com/intel-skylake-remove-support-usb-based-windows-7-installation-platform-specs/
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we have to admit one thing though, Intel has absolute plausible deniability on this. No one can claim with 100% certainty that they are in league with Microsoft in trying to push Windows 7 users to finally get a move on. The reason for that is that the
discarding of EHCI was inevitable sooner or later, and while they could have taken steps to remedy the problem easily, they have no compulsion to do so. The problem begins with the fact that Windows 7 does not have the code to support master xHCI based installation via USB, with only installation over EHCI allowed.
You might be thinking that one can always take the bootable DVD route and even though research indicates that dependence on optical drives is diminishing by the second, it has its own caveats.
You better hope that your motherboard has a PS/2 port because even while installing from a DVD, the USB ports wont work (during the install setup). If you have just one PS/2 port you will have to switch your PS/2 based mouse and keyboard as required through the entire process. All this means that Windows 7 will be very very troublesome for people to install and in most of the cases, people will consider it not worth the effort. Even if you do get it to work, the lack of EHCI means there might be unforeseen compatibility issues in the future. Effectively, from Skylake, and thanks to Intel’s spec upgrade, Windows 7 is now officially an obsolete OS.
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if you want windows 7 you are gonna need to run and get a ps2 keyboard
Microsoft really really wants windows 7 DEAD