What have i done?

i have a 02 reg ford KA, 6 months ago i accidentaly put water in with the power steering fluid, no idea how much really! got the fluid emptied and changed a week later, it seemed fine although there is a grinding noise when i lock the wheel, or almost lock it, although now the steering has gone really light and feels like its going to skid even when turning a corner, any idea how to fix it and how much will it cost??