If you are using drdos or a true dos equivalent up to and including MSDos V7 (Windows 95/98/ME) you need to have the driver for the cd rom drive loaded in the autoexec.bat file of the boot disk.
To do this you need the autoexec.bat text file in your root directory of your boot disk to contain an entry to load the drive MSCDEX.exe driver - typically you wont need any parameters if it is an ATAPI cdrom drive (Most are nowadays) and of you have ample spare drive letters available - for more complex scenarios the command line switches can be found here (
Command-Line Switches for MSCDEX.EXE)