Why isn't there any response when pressing Fn+F7 for dual display?

After carrying out the following, Fn+F7 cannot be used for dual display. There's no response from the shortcut keys either.

1. Uninstall Norton Security 2005.
2. Windows Update (Express).
3. Unplug the AC adaptor.
4. Start the computer.
5. Press Fn+F5, Fn+F6, Fn+F7.
6. Repeat Step 3 for 2 or 3 times. When pressing Fn+F7, there's no response. Other function keys (Fn+F5, Fn+F6, Eject) gives no or very slow response.

To resolve this symptom, please connect the computer with the AC adapter and restart the computer.

Note: Please start the computer with the AC adapter connected. This way, the symptom will not occur even if the AC adapter is disconnected after the computer completely starts.