The bootmgr is missing error occurs when there is an OS installed on the computer. You need to do a clean install which will involve formatting the drive as part of the installation process. To boot from the CD/DVD drive you need to change the boot order in the BIOS and set the CD/DVD drive as first in the list. Then restart the computer with the Windows XP installation disk in the drive. You should see a message about booting from the drive. Something like "Press any key to boot from CD..."