First, make sure you are not using a USB Hub. Connect the BB directly to a USB port on the computer. Then right click on "My Computer", click "Properties", go to the "Hardware" tab and click on the button that says device manager and scroll to the bottom to Universal Serial Bus Controllers. Right click on each instance of USB Root Hub and click "Properties", click on the power management tab and make sure the box next to "allow the computer to turn off this device to save power" is not checked. Do this for each instance of USB Root Hub. This has bitten me more than once.
If that doesn't help then you may have had a bad device. You can also try other USB ports on the computer.
One last note, be sure that when you install the Desktop Manager and the OS version to the PC that the BB is NOT connected. If the BB is connected the drivers may not install correctly. Good luck