Window AC unit with Grounded Plug Adapter?
EASY Method: 1) Get a three prong grounding adapter with a green wire extending from the adapter. 2) Attach the green wire to the center screw that secures the face plate to the outlet. (The outlet is grounded to the box, and you are tying into that ground.) 3) Plug the adapter into the outlet. (If you have a circuit tester, confirm that the ground is working.) 4) Plug in the Air Conditioner. 5) Turn on AC. 6) If you think you did it right, sit back and enjoy the cool. If you think you messed up, sit back and watch for smoke.
Safe to use 2 prong adapter with microwave?
The third prong is there mostly for safety reasons. I would not recommend it. The ground prong is there for a couple reasons, one of which is to protect you and others from getting electrocuted if for some reason the microwave were to malfunction. Instead of you being the path to ground, the third prong provides a safer path to ground. The ground may also be required to allow the device to perform properly. It will probably work just fine with an adapter, but because of the safety issue I would not recommend it. To be safe you need to have a licensed electrician take a look at upgrading your outlet to a GFCI outlet. This could be costly, but it is the safest way to go.
Replacing a 2-prong with a 3-prong receptable without ground?
Even if you detect with a multimeter that the junction box may show continuity to ground there is a high probability that the path to ground was not installed to code standards, may not be capable carrying the fault current available at the receptacle location and may provide a false sense of safety. My recommendation is to follow the NEC approved method of installing GFCI receptacles in place of the two prong. You may be able to wire downstream 3 prong (nema 5-15) non GFCI receptacles that are protected by the GFCI and marking the receptacles as having no ground. 3 prong adapters provide no protection unless the tab gets connected to a grounded source. GFCI cord adapters are available http://www.amazon.com/gp/product/B000XVG... if you have specific locations or equipment you need to protect for life safety, and don't have the ability to install GFCI's. These compare the outgoing and returning current and shut down if unequal, but do not provide an actual ground for electronic static discharge.
Why is it unwise to disconnect the ground prong from a plug?
You will no longer have a ground. If something goes wrong, you could be electrocuted.