To ensure that a computer can boot from disc, the start sequence must be set in the BIOS of the computer in such a way that the drive is addressed first as the boot drive (start sequence CD-ROM, C, A for instance). In the case of an SCSI CD-ROM drive, this drive must be connected to an SCSI adapter with a separate BIOS in which settings can be modified accordingly. (This will only work if there are no IDE hard drives present, as these come before the SCSI adapter in the boot sequence.) 
When booting from a disc, you can only start an operating system that does not write to the medium, such as “MS DOS” or “Linux”. During booting Microsoft Windows 2000 and Microsoft Windows XP write to the medium from which they are being booted. This is not possible with a disc and so the process is canceled and the PC cannot be started.

Requirements for Booting From a Disc