Windows xp pro

Hi,
I have windows xp pro installed on my computer. I have a problem that is when I boot up my computer, I must have the windows disk in my cd drive before it will boot up. I tried to install windows vista and the problem was still there. I then reinstalled windows xp pro again and the problem is still there. I took it to a computer shop to have it fixed and they said it was a cd drive that was bad. They unplugged the cd drive and it worked fine for a little bit after I brought it home then it started doing the same thing again. I have tried to reinstall windows and everything works fine during the instalation but when I try to reboot it does the same thing. Please help me if you can. Thank you.