Are you using the cable that came with your phone? Are connecting the device to a usb port on your computer directly and not through a usb hub? Those are just two things that come to my mind that could throw a kink in the chain of things.
jajajjaajjaajja!!!!!!!! guess wat i had same issue and i read so many thing and i try a few the succesful one is jus plug and unplug ur bb from the usb wire a few time its a reboot driver it works
Hope this helps somebody. I was frustrated by this while trying to charge with the usb cable on my laptop or downloading audio books. I'd been using an USB cable that "fits" but it wasn't the one that came with the phone. Tried the one that came with the phone and "no mas problemo"!!! Again, hope this helps.