Sorry if you dont understand I will try again. 1 put you XP CD in and let it boot the CD. 2 follow the instructions on installing windows XP. 3 when its finished installing and asks you for your CD KEY you could try putting in something, what should happen is it will say invalid KEY and take you to another page with the phone number on to ring. If it does not then ring this number 0870 60 10 100 UK number. 4 tell them you have the CD but not the CD key, tell them you reformatted the computer and that when you reinstalled windows you found that you did not have one.
They will do two things 1 tell you some instructions that you type in and then they activate it for you or 2 They will refer you back to the shop you bought the computer from.
If they tell you to go back to the shop then thats in your hands to challenge the retailer who sold you your computer,the only other way to do it is buy a new copy of xp to get the key or buy a new key direct from Microsoft.