I've a Toshiba notebook with Windows 8.1 and I need to install a driver for a hardware but the driver has no signature. I restarted windows as described in so many articles and got the screen below. But I cant select the 7th option, I tried "F7", "7" on both sides of the keyboard, plugging 2 different USB keyboards but no luck. So far only power key is working at this screen. It turns off the computer and it boots normally when I start it again.