You will need to have your DOS driver loading in config.sys and mscdex.exe loading in autoexec.bat. If you don't have a DOS driver disk for your CD-ROM drive, do the following if you have an IDE CD-ROM drive:
Make a win98 startup disk from add/remove programs on another machine, or one the you may already have. Copy oakcdrom.sys and mscdex.exe from the diskette to c:\.
Edit c:\config.sys to include following line:
device=c:\oakcdrom.sys /D:IDECD1
Edit c:\autoexec.bat to include following line:
LH c:\mscdex.exe /D:IDECD1
Boot the computer and hold the CTRL key down after the keyboard is detected (start tapping F8 if using win95). Choose "Command prompt only". At the C prompt, type in win/d:m to start Safe Mode. This will let you access your CD-ROM drive in safe mode, it's not going to let you burn a CD if that's what you're trying to do (backing up before a format?).