Big windows xp backup problem

You can't.
Make sure you know your XP home serial number and write it down.
Find someone with the same version XP Home edition and you might be able to use that disk to install. When it asks for the serial number, you supply yours that you have written down.
Now I say might because some computer manufacturer's create a restore partition on the hd and you might need a restore/recovery CD supplied by the manufacturer to reinstall windows.
It would help if you posted the brand and model of computer.