To boot off the XP disc, you need to boot into the BIOS, and set the CD/DVD drive as the first boot device. To do this, tap the delete key repeatedly as the PC is booting. Look for the first boot device setting, and change it. Make sure to save the changes and exit (usually F10).
I did that but my bios won't stay on the screen long enough for me to change anything. The error was a unmountable boot volume. I got info to disconnect and then reconnect the hard drive. While I was inside my tower I also cleaned all the dust out. Now my hard drive won't even boot at all! Do I need a new hard drive?