If as described you have BX cable (armored Cable), you would need to know if there is a bonding strip/wire in the jacket. If it is old style BX, then there will be no bonding strip and you cannot use it for grounding.
Your best bet is to add a new circuit with current style wiring. Adding a ground wire only is almost as tough as a new circuit all together, so just run the new circuit.
In addition, you should switch back the two prong receptacles, as it is not legal to just replace them with a three prong, unless you install a GFCI receptacle at the beginning of the circuit to protect the down stream. Then you would put a label on each downstream three proong recept indicating it has no ground.
[This message has been edited by Ron (edited 07-27-2003).]
