Many times when building a base system to image, some start with the OEM installed OS that came with the system. This always creates a problem when you sysprep because you cannot simply replace the license key with a select key since it is an OEM preactivated install.
OEM Installed and activated Windows XP Pro or Windows 2000 system you want to capture an image of for distribution to other machines.
A Select license key is not valid for an OS installed using an OEM install source. The key is different for each machine you receive.
This article from Microsoft explains what to do.