Computer games and kids

My son is very addicted to the computer and his computer games. They seem to take priority over everything else in his life. He would rather play a game, then come help me with any tasks (which I can understand to a point). I find he almost zones out when he is playing his games and often I have to repeat myself several times before he actually hears me. My youngest daughter hardly ever plays the computer and she is a much happier child. I don't know if this has anything to do with the games or if it's just the way their personalities are. I do try to set a limit on how long he is permitted to play on the computer though.