Keyboard problem?

Hi,

My sister has a laptop and i'm sure many of you will be familiar with the function key of Fn. As you may also know that where some letters are there is also the pc equivalent to the numbers on the left. Well when we press the M key it displays a 0 and when we press Fn and M it displays an M, obviously this should be the other way round and we don't know how to do it. The light at the top is on but havn't got a clue how to turn it off. It is a Dell CPX, anyone help ????