Its not booting off the CD, it should ask you to press a key to boot off CD.
It shouldnt matter which OS CD you use, it should still go to a screen that asks you to press any key to boot of CD as it will do that before it even accessess the hard drive, regardless of which OS is actually on the hard drive.
I think that you have a faulty XP CD or for some reason its still set in bios to boot off the hard drive before the CD.