Boot Media Error

I am trying to install XP professional on a system that has Home installed. I pop in the Windows XP cd, and the autorun starts, and I could install. I want to format, so I stop the menu, reboot. Upon rebooting to cd first, i get a “Boot Media Error” and have to C-A-D. I take out the cd, it boots fine. Put it back in, get the error again. What is on the cd that the Boot process is not liking?