I've just been through all this as my computer suddenly decided to stop recognising all the USB devices. Have done ALL the things above including twice deleting all references to USB hardware in system manager and the registry and rebooting whereupon windows then reinstalled all the USB hardware. It STILL wouldn't work.
Then I remembered the schematic of a modern PC power supply. Turning the PC off doesn't really turn it off as the motherboard is still powered. So I pulled out the power cord, waited 30 seconds and put it back in. Et Voila! USBs all working again.
My theory is that the USB driver has an internal microprocessor and it needed a reboot. I wonder how many PCs get taken to the repair shop with this problem, are unknowingly 'fixed' when the power cord is unplugged, and the repair guy gets all the credit for fixing the problem without having to do anything!