I was in the middle of installing Windows XP in VMware Server when it hit me that I had no idea where the product key was. I decided to mount the install ISO and browse around and sure enough I found the file unattend.txt in one of the sub folders. This file is used by a program called Sysprep to automate the Windows install process. To find the file, use the following command:
awk -F= '/^ProductKey/{ print $2 }' `find /mnt/iso -name 'unattend.txt'` | tr -d " \""
This assumes your install media is mounted at /mnt/iso. If the install media is mounted somewhere else (such as /media/cdrom0 for an install DVD), simply change the path for the find command. Hope this helps!