Weird problem typing code

Hello, I have a strange problem when I try to activate windows XP home edition:
When I try to type the key code to activate XP, The letter 'Z' will not show up on the box when I type it. All the rest of the code digits will appear normally except the letter Z. Since its part of the code, I am unable to activate windows. Do you have any idea what is causing this , or how should I fix it?
I tried installing the windows again but it seems to be a permanent problem On my PC
Thank you