SDL keyboard arrows and numpad not working

I have a program which react to keyboard inputs. It works well on most computers
but on some computers it’s not working (same OS: XP, same keyboard language:
french belgium).
I’m tracing the value of the keysym (event.key.keysym.sym) and on the computer
where it’s not working I receive 0 for all arrows and 273 for numpad key 8 (273
is the value for up key SDLK_UP) instead of 264 (SDL_KP8).
Any ideas?