Re install of windows XP

I have taken the drastic action of wiping my laptop clean and starting again, but when i tried to reinstall windows xp, when it comes to the part when you have to type in your product key number then about 4 letters on the keyboard which now strangely do not work, with the B and the W being in my key number, i can not reinstall…..why do only these letters now not work….any ideas on how i can get my laptop working?