I've posted about such a thing in this answerin this answer.
To understand what is going on, refer to this tutorial.
Here is the full list of scan codes from Mirosoft so you can alter any key.
Here is the basic breakdown. You are editing a registry key. This particular one is set to map Numlock (45,00) to null (00,00) so that it's key does nothing. For some odd reason, re-mapping 1 key means you need to specify a 2 (02,00,00,00) in positions 17-24. Likewise if you map 2 keys, you'll need to specify 3 (03,00,00,00).
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Keyboard Layout]
"Scancode Map"=hex:00,00,00,00,00,00,00,00,02,00,00,00,00,00,45,00,00,00,00,00