You may or may not be getting CDRom support with the boot diskette. There are a few CDRom drives where the Windows boot diskette isn't able to provide you with support. In that case you either need to load the real mode CDRom drivers or use something like the freeware program, CDGod, to create a boot diskette. The easiest way to explain this is ......
Boot to your W98 boot diskette. You should see a three option menu appear on the screen. Select the option for "Start with CDRom Support". Then as it's attempting to give you CDRom support, somewhere on the screen at the very end, it tells you what drive letter it has temporarily assigned to the CDRom drive if it was able to provide CDRom support. It will look something like this, "Drive X =MSCD001", where "X" is the drive letter that's been temporarily assigned to your CDRom drive for the install. If you don't see that phrase on the screen then you didn't get CD Rom support. Assuming you did see that phrase on the screen, insert your Windows CD and type the following at the a:\> prompt. Replace the letter "X" with the driver letter that the boot diskette assigned to your CDRom drive.
X:\setup