Ok, now that we have established that the disk won't boot the computer, this could mean about 3 different things.
A) A BIOS boot setting isn't set properly, but considering you installed XP using a CD previously, your computer isn't booting from that CD now and you probably haven't changed those settings since then, as well as the fact that the CD drive is also not working in windows, this is probably not the case.
B) IDE cable failure. When you bought the new cd drive to test out, did they give you a new IDE cable (the wide, flat, probably grey cable) to use, or did you use the old one? If they gave you a new cable, then this is probably option C. If they didn't, I would suggest buying a new IDE cable for around $5 and testing the CD drive with that first.
C) IDE Slot failure. This is where the IDE cable plugs in on your motherboard. I do not know exactly how your settings are inside your computer, but they would likely be one of two things. Either:
-CD Drive and Hard Drive are connected on the same cable in the same slot
-CD Drive and Hard Drive are connected on different cables to differen slots
If your hard drive and CD drive are connected via the same cable, then it is probably cable failure. If however, they are connected by different cables to different slots, then this would probably be a slot failure, in which case I will tell you how to connect them both on the same cable and the same slot. Let me know in your next post which one of these it is.
In regards to the whipper snapper comment, no offence whatsoever taken
. Just trying to put my knowledge to use by helping other people.