My moms computer died and I just built her a new one. I went to install her copy of Windows XP Pro and we can't find her product key She doesn't want an illegal copy. Is her only option to spend $300 and buy a new copy or is there some way to find out her old product key?

Keep in mind the other computer is nonfunctional. If I called microsoft in the morning would they be able to look up her old product key?