Dual boot XP on an existing Windows 10 installation.

Looks like CD/DVD inserted is not bootable.

DavidY said:

Have you turned off Fast Startup in Windows 10?
Turn On or Off Fast Startup in Windows 10

If Fast Startup is on, then when you 'Shut Down' Windows 10, it actually does a form of Hibernation, which I think means the BIOS doesn't let another OS boot.

That's a good point, but a restart (reboot) is not affected by Fast Startup. I would turn it off, in any case.

Also, check your BIOS setting for boot sequence once more. I know you said you set it, but if you accidentally exited with a save of the settings, it did not take. Just check it one more time to be sure.

Also a question. This DVD.....is it an OEM XP Install Media? If it is and it did not come with your current machine (i.e. matches the key in the MB), it will not work. OEM Install media will not work on different machines.

Samuria said:

Has your pc got secure boot as that would stop it. You always install the oldest o/s first as it wont understand the boot for new o/s so if you install xp after 10 things wont boot you will have problems. Why not run it from a virtual pc with virtualbox which is free Downloads – Oracle VM VirtualBox

Another good point. I had the same issue when I tried to put Win 7 on after 10. It got all messed up in the BCD. Still, if that's an OEM XP media, he's gonna have problems with the license.
@idigweeds , do you have any other bootable DVD, like a Windows Rescue or anything you could try to see if your BIOS is set right and will boot from any DVD?

idigweeds said:

?MS makes unbootable OS install disks?

May have been bootable once upon a time but CDs are not most reliable media.