Are you entirely sure that you've got no USB devices in your system:
internal flash memory card reader, USB mouse and/or keyboard may be good examples.
On the other hand open up your case and check your motherboard USB headers. Make sure that all plogged in connectors are snug (these are used for case extensions - i.e. USB connectors on the front of your computer case).
Re-install of the operating system may be required. If all else fails disable USB in your BIOS and get PCI USB add-on card.
Good luck.