At exactly 0 current hit points, or in negative hit points but stable and conscious. A disabled character may take a single move action or standard action each round (but not both, nor can she take full-round actions). She moves at half speed. Taking move actions doesn't risk further injury, but performing any standard action (or any other action the DM deems strenuous, including some free actions such as casting a quickened spell) deals 1 point of damage after the completion of the act. Unless the action increased the disabled character's hit points, she is now in negative hit points and dying.
A disabled character with negative hit points recovers hit points naturally if she is being helped. Otherwise, each day she has a 10% chance to start recovering hit points naturally (starting with that day); otherwise, she loses 1 hit point. Once an unaided character starts recovering hit points naturally, she is no longer in danger of losing hit points (even if her current hit points are negative).