A locked camera does not allow pictures to be downloaded to PC. There are 2 methods of unlocking:
1. Sending a $80 getkey USB command followed by a $80 sendkey with a response key based on key returned from getkey. This works for old cameras that share a single key. Newer and Older cameras can be "reset" to a simple 1,2,3,4,5,6,7 key by following a specific procedure documented in these threads. There are currently only 2 known response keys - one for older firmwares and one for reset cameras. This unlock needs to be performed every time camera is hooked up to computer.
2. Change firmware to permanently unlock camera. This method is more complicated and not for dummies. Even when followed correctly it has "fried" cameras. So far there is no method to recover a fried camera.