You need a Windows 98 Boot disk (floppy)
Do you have one?
Do you have access to a Windows 98 machine to make one?
You can get one here http://bootdisk.com/original.htm
Set your BIOS/ setup program to boot from floppy first.
Start without CDROM support, type fdisk
after answering "yes" to enabling large drive support,
select the option to display non-dos partions( just to be sure XP didn't make one)
Then look at your DOS partitions and be sure you have a "C" and it's set as active.
If everything looks ok, exit fdisk, control/alt/delete to restart...
This time start with cdrom support , and you should be able to switch to your cdrom letter and type setup