try this;
1. make sure the coin is in an MC;
2. place the MC on the main stage.
3. right-click on the MC and select actions
4. enter;
Code:
onClipEvent(load) {
   leftMargin = 100;
   rightMargin= 300;
   topMargin = 100;
   bottomMargin = 400;
}
onClipEvent(enterFrame) {
  if (_parent._xmouse > leftMargin &&
      _parent._xmouse < rightMargin &&
      _parent._ymouse > topMargin &&
      _parent._ymouse < bottomMargin ) {
     _x = _parent._xmouse;
     _y = _parent._ymouse;
  }
}
change the margin values to suite your needs
hope this helps
BlinkOk