Problem: When you try to use the number pad the mouse pointer will jump to another part of screen and anywhere in the sentence you are typing...
If the Num Lock is On, but the number keys on the numeric keypad are not working as expected, then one possible option you can check if the option Mouse Keys is turned on.
If you have Windows Vista or Windows 7, goto Control Panel-> Ease of Access-> Ease of access center. Then scroll down and look for the option "Make the mouse easier to use"
If found click on it and look for the option Turn on Mouse Keys.
When Mouse Key option is turned on, the default operation of numeric keypad is disabed and it is then used to control mouse pointer.
Turning off Mouse Key resolves the problem.