I use MS Paint for my diagrams. You've got all the tools you need to do it. You kind of have to drag your items around yourself, and then re-draw lines to each object. But it's still doable.
Those other software options mentioned here would probably be quite a bit more suitable. But if all else fails, you still have MS Paint.
Those other software options mentioned here would probably be quite a bit more suitable. But if all else fails, you still have MS Paint.