Computers in Preschool: Hurting or Helping?

Because computer knowledge is vital in our society today, many parents believe that the earlier their children begin to use the computer, the better. By the preschool years, most children are spending time on computers at school and/or at home. But is it healthy for preschoolers to be interacting with computers? And if so, how do parents decide how much computer time is beneficial or when it's too much?