[kactuskid]

Hmmm, you need to hold off installing those 3 prong receptacles until this can be figured out. If you have the older cloth romex with no ground wire, then you may not really have a ground. But wierd that you would read the voltage then.

Are there any other wires in this box that are connected to the box or making contact with it? Did you disconnect the wires from the receptacle, spread them apart and then take your reading? If you just measured from the front of the receptacle to the box, then it's possible that you may have a bootleg ground somewhere in that box or on the circuit. This is where a ground and the neutral are bonded together on the load side of your electrical service panel and it is very dangerous and can give a false reading of having a ground.

One way to know for sure if you have a bootleg ground is to connect a GFCI receptacle to the wires in the box. The GFCI will sense a ground fault and will trip if anythings plugged into it.