I'm pretty sure I ran into this problem after Acronis Backup & Restore 11.5 imaged my disk to a .tib file.
Both the source system and the target system were apparently missing C:\windows\inf\usb.inf afterwards.
I copied it over from my netbook and an 'Update Drivers' later, had my logitech k400 wireless keyboard working again.
This seems to have affected not just logitech unifying receivers, but all USB Composite Devices.
My Android tablet is also affected, as well as my Microsoft Xbox 360 USB Controller receiver, since it has two 360 pads sync'd to it, but my wired PowerA Minix360 is not affected.
My microsoft wired keyboard 600 and logitech V220 are also not affected, as they are not 'USB Composite Device', but 'USB Human Interface Device'.
USB Composite Device is used to wrap up multiple child Devices in a single parent Device, such as two USB Human Interface Device endpoints (One for keyboard, one for mouse) like the Logitech Unifying Receiver does.
Big thanks to lucvdv of answers.microsoft.com for figuring it out:
Some time ago, my system stopped recognizing some, but not all USB devices.
USB ports are fine, it reads the device descriptors but then either says
it's unknown hardware or says it can't find a driver for it.
A second symptom that must have started at the same time is that it
"forgets" all about some USB devices that have always worked fine before,
with the same result: suddenly either "unknown" or "can't find a driver".
Two weeks ago, it was my keyboard.
It still worked when I logged on (it had to, I used it to enter my password).
Immediately after logging on, the found new hardware thing popped up,
after a few seconds it said it failed to install my new hardware, and at
that moment my keyboard stopped working.
Never found a solution, I've been using a PS2 keyboard instead since.
I've tried other USB keyboards, it didn't recognize any of them.
Yesterday, it was the USB hub that's built into my monitor that
suddenly turned into an "unknown device", and nothing I connect
to it is detected anymore.
In an attempt to fix that, I launched the device manager and let it
scan for new hardware.
Result: instead of my USB hub starting to work again, my
Logitech wireless mouse stopped working.
The "USB receiver" is now listed with an exclamation mark,
"drivers for this device are not installed".
It worked fine just a few seconds before.
Right now I'm back to using a wired USB mouse, but I wonder how
long that's going to keep working.
It's not the receiver that's at fault: when I plug it in,
it shows up in device management with the right name and device ID:
USB\VID_046D&PID_C52B&REV_1201 - USB\VID_046D&PID_C52B,
and status "The drivers for this device are not installed. (Code 28)".
Comparing the contents of %SystemRoot%\Inf to another Win7-64 system,
I noticed that file USB.INF was missing.
Copied it over from the other system, started device manager and
re-installed drivers for the non-working devices, and the problem is fixed.
Thanks for your patience in tracking it down, lucvdv!