Lessening screen time with teens?

<p>My kids are 15, 13, 11, and 8.  The TV isn't a huge problem, we don't have cable but have a Roku to watch Netflix or Hulu occasionally.  My problem is their computers/games.  Their dad bought them laptops/Nintendo DS's, and especially my teens have become addicted to the pc.  They want to be on it all day, especially now during the summer.  It's driving me bananas.</p>