Been there, done that. :)
Okay so do you have any method of input to Windows? If not, how do you shutdown or restart? Do you have a serial mouse (assuming the system has a serial port) or something available? How about another USB keyboard or mouse (specifically a different make, model, etc.; something that Windows does not already know about)?
If you do have an alternative input device (frankly, either mouse or keyboard is fine since you can set up either to do the work of both), then use that. Windows should detect the new hardware and install it regardless of what the data in the HKLM\SYSTEM branch used to be. If you don’t have an alternative input device, try plugging in your USB keyboard/mouse into a different USB port. Windows should still detect a new device and install the drivers and configure the new port.
If you now have the ability to use a keyboard or mouse, then you will have to open up Device Manager and look through the USB devices that are giving you trouble. The easiest thing to do would be to select Show Hidden Devices in Device Manager, then unplug the device, then uninstall the device (in Device Manager), then plug it back in. That way Windows should reinstall the drivers cleanly.
If there is still problems with the devices, then it will probably require some more, unpleasant work to fix, though you you shouldn’t have a problem since you restored a backup of the registry rather than fixing the corrupt one.