This was really annoying me until I found the cause of the problem……
I have a virtual machine running Ubuntu 10.10 (Maverick Meerkat).
They keyboard has always worked perfectly until some updates a few days ago.
After running the updates, I found that my numeric keypad had stopped working.
To fix this, click System > Preferences > Keyboard
Select the Mouse Keys tab and uncheck the “Pointer can be controlled using the keypad” option.