If you are using Xfce, you can add numlockx to your "Autostarted Applications." Go to the main menu, then "Settings," then "Autostarted Applications." A dialog box will appear. Click "Add" and type in "numlockx" where it says "Command:". You'll also need to give it a name. If you do this, numlockx will start when you log in (this assumes you have numlockx installed).
This may not be the best solution, but it should work.