Windows XP reboot at loading screen

Hello,
i am having a problem with my computer when i start it up it starts up fine until it hits the windows xp loading
screen so i thought i could try re installing it but when i boot from cd my computer just stays at boot from cd/dvd for ages or it just stay there for a few seconds then carries on into the windows loading screen and just skips the cd!! so i thought id put the hard drive into a different computer and reinstall xp on it and put it back into the computer but the same thing happens please if anyone has a answer to this i would be extremely grateful thanks