This problem may be caused by a Hidden Device try uninstalling the Hidden Device and restart the computer.
Type Device Manager in the Search box above the Start button and press ENTER. Are there any yellow question marks in Device Manager? If yes what is the Device Error code.
To see Hidden Devices (Non Plug and Play Drivers) you need to select View, Show Hidden Devices. It is necessary to do this every time you open Device Manager if you need to see them.
A further complication can be Ghosted Devices –installed but not connected!
http://www.lockergnome.com/windows/2005/04/29/display-hidden-devices-in-device-manager-xp/