There are several options you should check.
1. If you are trying to boot from XP CD on the computer with no
Windows installed,
then you should check BIOS settings for boot order.
Usually it is done by pressing either DEL or F2
(you should choose what you you need by looking at the screen during
the booting)
and in the upper BIOS menu choose the option - Boot Order.
Usually it is called something like that.
In the boot order you should put CD or DVD (it depends upon the type
of your cd-dvd drive)
on the top position and then save BIOS settings and restart the PC.
Of course, boot Windows XP CD should be inserted before rebooting.
2. If you are trying to boot from XP CD on the computer with
preinstalled Windows,
and trying to autostart Windows XP CD from the Windows then the
possible reason
is that the autostart CD option for some reasons is disabled.
But you can manually start CD by opening this CD in Windows Explorer
or any file manager you are using,
find the file Setup.exe in the root directory and start it. That's it.
3. It is possible that you may have problems with your boot XP CD.
Try to clean the working surface carefully with special liquid if you
have
and check it for scratches.
In any case feel free to contact me.