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Starting old first.
If you have Knob & Tube wiring, ideally you should reconsider rewiring the home. But if that is not an option and you need a few grounded outlets. You will need to run another green ground wire
If you have a older cloth covered Romex wire system the ground should already be there inside the box. You need to look inside for the bare copper wire that is twisted together and then oftentimes run out of the box within the wall cavity and screwed to the box. If this is there, you simply insert a grounding screw into any of the remaining screw openings within the box and run a short ground wire to the outlet itself.
If you have BX wires normally this is the same as the older Romex wires. Look for that little ground wire coming out of the cable and that is attached to the inside or outside of the metal outlet box. If you see it, simply do as the Romex one. Run a wire from box to outlet.
If you do not have that extra bare ground wire that is connected to the box you have one or two options. Run a wire as with the Knob & Tube wiring, or wire a GFCI outlet and mark it GFCI protected ungrounded. Do not rely on the bx exterior sheathing to provide a useful ground. Many have and lost their homes when the metal outside began to glow when power went through it because of a short.
A lot of what you do will depend upon what the reason is that you need a three prong outlet for. If its simply for looks, and the outlets are for table lamps and such, not a real big concern, However if you need to do this for special Holiday Gifts such as that new 60 inch TV, I would strongly suggest that you either run a ground wire to the box or a new wire completely to assure that your equipment is properly grounded.
I agree that the AC metal cable jacket usually is not installed properly and is probably not a good grounding path back to the panel.
So, to answer the original question from John S, If there is a good ground in the box you can put in a three prong outlet but it MUST be properly grounded. If there is no good ground then you either have to provide one from the the nearest grounding means or us a GFCI receptacle and mark it as ungrounded. Any receptacles down line and protected by that GFCI may be replaced with 3 prong but MUST also be marked ungrounded.