2000 Cougar loses power

My wife has a 2000 Merc Cougar with the 3.0L V6. She was driving to school this morning and started to lose power to her dashboard, radio, and power windows. When she stopped at a light she also lost most of the power to her engine. She made it to her parking space, parked the car, and the engine began to rev up and down on its own. The car then shut down completely and when she goes to try and start it she get sonly clicks. I have heard that a shorted battery or bad alternator can cause these problems. Any help would be much appreciated.