Did you make sure that you've changed the boot sequence in the BIOS to boot from the CD Rom first when you tried to boot from the Windows XP CD. If not, set the first boot device as the CD Rom and then put in the Windows XP CD and then save and exit, usually F10 (your computer should now reboot, if not reboot manually). When you see the "press any key to boot from the CD" screen, do so and then follow on screen instructions. I would do what CrazeD said and see if you can repair your installation of XP. Check out the link below for a more detailed explnation.
EDIT: CrazeD, I wasn't concentrating. Bit of a heavy night last night, lol.