This morning, I set about installing the Windows Phone SDK on a Windows 8 64-bit desktop. The emulator relies on Hyper-V, which in turn relies on the virtualization support of the central processing unit, which, in this case, is an Intel Core i7. After rebooting into the BIOS and enabling virtualization, I found that I would consistently have no keyboard and mouse available in Windows 8. If I rebooted and went into the BIOS settings, I would have both. Yes, the mouse, too: the motherboard is an Asus Maximus IV Extreme, which, in addition to such insanely virtuoso capabilities as permitting one to overclock the thing from an iPad, allows one to navigate the BIOS settings with both a mouse and a keyboard. It was only after Windows 8 began loading that all of my USB devices would disappear.
I toggled the virtualization support on and off and confirmed that it was indeed having that setting on that led the USB devices to not be detected. And yes, I ensured that I had flashed up to the latest BIOS.
When I set about finding a solution, I discovered that it was not just developers like myself that were encountering the disappearing devices problem. I gather from TechNet that, especially of late, folk are having the miserable experience of turning on their Windows 8 laptops and desktops and finding that they have no keyboard or mouse control. Only those among them with touch screens are able to log-on!
Someone had posted a fix to the TechNet thread, which seems to also work for my case. In the Control Panel, under Power Settings, select the option of choosing what the power button does. Disable the "Fast Startup" option.
Of course, I was able to get my mouse and keyboard back by turning off virtualization in order to be able to manipulate this setting. I'd be very much inclined to recommend disabling Fast Startup on any Windows 8 PC while one still has devices to control it.