Patching and Plastering - Loose plaster on wallboard

I recently noticed a spot on the wallboard under a bay window in my house that looked like it had been patched. I touched the plaster and a whole foot wide section would move to the touch. I think if I put any pressure on it I could probably stick my hand through it. I am guessing it was water damaged and they just plastered over it to cover it up. I was wondering what the best way is to repair this, and if anyone has any ideas how the damage could have occured so I can make sure it doesn't happen again, or still isn't happening.

thanks

Roscoe_Burdick, Start at the top, check the roof of the bay. The water can come in at the top and run down the inside of the wall. Also check the windows at the bottom to make sure any caulking and flashing if any might be the problem. All I can do is tell you what I would do if this was my problem. I would take out the sheetrock between the studs where it needs to be replaced. Take a light and look to see if the area is still wet. If it is put a fan on it to dry it out. Once dry take garden hose and starting at the lowest point outside flood the windows sills. Wait a while to see if you have any new water. If not move on up with the water until you find where it is coming in. If you don't find any leaks I would wait until it rains a hard rain and check for water. It may be that the leak was repaired and a poor job of repairing the wall was done. After leak ifany is repaired put in the new drywall and fill it and you will be ready to paint/paper...Mike