It will stop with an error message saying something like "No operating system found", then you can insert the XP disk and reboot. Or, if you are quick, you can open the CD tray and put the CD in as soon as it starts, and not wait for the error message. You may first need to go into the BIOS setup and change the boot order so that the machine boots from the CD - there should be a message during the early part of the initial boot saying something like "Press Del for BIOS setup" and you do what it says.
