Make sure that the USB connection mode of the camera is set to [Mass Storage], or that

is displayed on the task tray.
If this is not the case, select [Start] - [All Programs] ([Programs] for Windows(R) 2000) - [Sony Picture Utility] - [Tools] - [Media Check Tool].
If a network drive is set for your computer, select [Tool] - [Map Network Drive], and select an available drive starting from the last character (Z:) of the alphabet for the network drive. Then, re-start the computer.