A two-prong outlet doesn't have an electrical ground. It's just hot and neutral. If the house is wired correctly, neutral is at ground potential, but it might not be. If there's knob-and-tube wiring, there's no ground at the receptacle box. If there's BX, there might be a bonded connection through the jacket of the BX connection, but that's bonding and not grounding. An upgrade that adds grounds to a water pipe isn't adding a ground; it's adding a connection to a water pipe. The pipe may or may not be at ground potential, depending on if it is continuous to an earthed connection. Tying grounds to a pipe is one of the ten worst grounding mistakes you can make. The NEC disallows the use of a pipe as a grounding point unless the attachment is within five feet of the grounding rod connection.