In a heavy 2 quart saucepan, over medium heat, bring to a boil sugar, corn syrup, salt, and water. Stir until sugar is dissolved. Set candy thermometer in place, and continue cooking. Stir in peanuts (if using roasted peanuts, add at 250 degrees F). Stir frequently until temperature reaches 300 degrees F (150 degrees C), or until a small amount of mixture dropped into very cold water separates into hard and brittle threads.