You could always go the long way about doing it.....If you have a jump drive available.
First you would find all devices installed on the computer. Write down all device names, then go to proper manufacturers website. You would download the recent drivers for their hardware. Save those files to the jump drive.
Once you have all files downloaded, you could use your XP cd, but only the XP cd. You would load a fresh install of Windows XP on their system, completely erasing EVERYTHING on the hard drive. You would need their windows XP serial which should be found on the side of the computer tower.
After the fresh install, you would start loading the new device drivers onto the system via device manager. After all is installed, then connect to the internet to start the upgrading process for Windows XP. Depending on the installation disk that came with your computer, you might be all night downloading new service packs that Microsoft had come out with.
I would suggest taking it back to the person that normally does the service for them. He knows the system. Unless you feel like going through all of the hassle of what I mentioned.
Now this is by far not the easiest way, but would completely erase all info from the hard drive and allow the owners to start over "new".