How do i delete a windows xp system from my computer?

[gemma m]

I have just added an additional windows xp system to my computer as i could not delete the old one or over right it as some of my keys dont work on my keyboard! Im selling the laptop so would like to get rid of the old system with my stuff on it. The desktop on the original system is not visible - so ive deleted things by using [Ctrl + Alt + Del] but i cant see my recycle bin to completely delete the files. Any ideas?
Ive already used the cd that came with the laptop to try and over write the one i had but in order to delete the old one and over write with a new one i needed to press the button 'L' but that doesnt work on that laptop! So i made an additional system - but the old one is still there. Is there any way of keeping the new one and just deleting the old one. I can select which one i want to use when i start the computer.