Why is my numpad acting like an arrow pad? (Aluminum Mac keyboard - wired)

I'm using an Intel iMac Core2 Duo and I've just attached a *brand new aluminum wired Mac keyboard* to it.

My problem is: the numpad at the rightside of the keyboard does not function as a numpad. Instead, +it's functioning as an arrow-pad+! When I tap "4", the cursor travels leftward. When I tap "6", the cursor travels rightward. When I tap "5", the it activates the cursor on screen. When I tap "9", my cursor travels diagonally rightward.

I simply can't get my numpad to function as a normal numpad! I've tried plugging in an older Mac keyboard (white keys and clear plastic), and the same issue arises. So I guess it must be an iMac issue?

I've tried searching through my system preferences for the mouse and keyboard and nothing seems to be amiss....

What do I do? It's really quite annoying, because I want a normal numpad!