A PC can only be booted from a CD when its BIOS supports this function (start sequence e.g. 'CD-ROM, C, A') or, if a SCSI CD-ROM drive is being used, the drive must be connected to a SCSI adapter with its own BIOS whose settings can be changed accordingly. (This will only work when there are no IDE harddisks present as these come before the SCSI adapter in the boot sequence.)
If neither of these conditions is met then the PC cannot be booted from a CD-ROM.
Even so, not every operating system can be booted from a CD. During booting, Windows95/98/Me and Windows NT/2000/XP write to the medium from which they are being booted. This is not possible with a CD and so the process is cancelled and the PC cannot be started. No provision for booting from a ROM medium was made when these operating systems were developed. When booting from a CD therefore, you can only start an operating system that does not write to the medium, such as 'MS-DOS' or 'Linux'.
Bootable CDs are created according to the 'El Torito' standard. The CD contains a boot image and an ISO part. When the read driver for the CD-ROM drive and the 'Mscdex.exe' file have been installed, MS-DOS can access the ISO part of the CD which runs the rest of the installation routine.