OK, if even one computer works your ISP will tell you it's your problem. Try resetting the AirPort Extreme as well. Perform a "hard reset" according to the following excerpt from Resetting an AirPort Base Station or Time Capsule FAQ:
- Press and hold the reset button with a pen or pencil until you see the status light (LED) start to flash amber rapidly, which should occur after about five seconds.
- Release the button, and the AirPort Base Station or Time Capsule will hard reset.
You will need to configure all its settings again as if it were new.