What discs do you have?
Do you have a system restore disc from the computer manufacturer or do you have a Windows installation disc for the XP operating system that came on the computer?
You will also need the key number (license number) of your Windows op system.
If you can boot your system and get on the internet, there is a freeware program that will give you your key number:
Magical jelly bean keyfinderhttp://www.magicaljellybean.com/keyfinder.shtml