so if u have the original windows xp cd still with u, then open nero (not the wizard but the other one. then in the recorder menu, select image recorder. after this, select to burn cd and it'll prompt u for a location where u want to store the image. let it complete and u r done, it creates an iso image for u which is an exact copy of the cd that u used. so if ur original cd was bootable, then u r good to go. then when u want to burn this image onto a cd, select recorder and select ur actual cd burner in the list. then select burn image and select the image location and burn it onto ur disk.

if ur original disk wasn't bootable then in nero there is an option to make a bootable disk but as i said before it might not work correctly. hope it helps.