USB num pad (combined with mouse) overrides NumLock on laptop keyboard

[Adam Funk]

I'm running oneiric on an Asus laptop with a normal laptop keyboard
(i.e., no separate num pad, but a "special" key that makes one of the
F-keys turn num lock on, turning part of the main keyboard into a num
pad).

I just got a Hama 2in1 USB combined mouse & num pad.  The mouse
functions work fine.  The num pad works too, but when I press any of
the buttons on it, the num lock function on the main keyboard is
activated, so I can't alternate between entering words with the main
keyboard and numbers with the external num pad.  In order to
deactivate the num lock function on the main keyboard, I have to
unplug the USB device and then use the special-num-lock combination to
turn the on-board num pad off.  (I hope that makes sense.)  Using the
num-lock control while the USB device is plugged in, after any of its
keys have been pressed, has no effect.

Is there anything I can configure to completely disable the num pad
functionality on the main keyboard, while still allowing the external
one to work, so I can use this set-up the way I want?

(I touch-type, including using the num pad on my desktop for anything
over 3 or 4 digits, & I'd like to be able to work the same way on the
laptop.)