abort XP Home re-installation

A friend of mine tried to fix a system crash. Instead of repairing it, he reinstalled the operating system. When it came to entering the product key, he then realized he lost it. Now I’m trying to abort the re-installation and fix the original problem so that I can retrieve the product key. It wouldn’t let me. I have deleted all new files in the root directory and windows directory thinking they could be flags for the system to tell it is being installed. However, it still asks for the CD for continuation of installation every time the system is booted up. I wonder if somebody can tell me how. Thanks