Im running Windows 7 and sometimes my number bad and arrow keys will, when pressed, perform a totally different operation then their standard function. For example the 2 on the num pad functions as a back button in both internet explorer and firefox browsers, and the down arrow will collapse a window, regardless of the program.
Check NumLock status, and are you running any type of 3rd parth software from Logitech or such that can remap your keys? Also some features of assisted navigation will do things, make sure that is not on (those are the features people with physical disabilities can use to navigate the computer easier).
I too have a similar problem. But unlike you, the 2, + and / keys on the numpad don't work at all and neither does the down arrow. I'm running Windows 7 on Intel Core 2 Duo. The NumLock works perfectly fine for the other keys.
One more thing - my AVG Internet Securities v11 keeps giving errors and crashing.