Laptop keyboard not responding but external keyboard works ok

The keyboard on my Inspiron 2500 has stopped responding. No keys function but the touch pad and click buttons are ok and the external USB mouse works ok.

When an external keyboard is attached I can type fine. My son states that “he was just using Word when the keyboard quit working.” The computer uses XP with all updates current. There is no apparent damage that would suggest the laptop was dropped. When using an external keyboard all computer functions seem to be normal.

Is there some utility I could use to test the internal keyboard? Any ideas on what would cause this and how I can fix it?