Do you have a floppy drive?

http://www.bootdisk.com/
get and make a win98se boot disk
Make sure it has FDISK and FORMAT on it.

Boot on it.
Run FDISK Delete all partitons, create primary DOS partition, REBOOT
Boot on Floppy again, run FORMAT C:
Reboot
Pull floppy and boot on Win98SE CD

Select Install Windows 98

After you are all done, you need to put on any mainboard drivers, device drivers etc, such as audio, video card, touchpad drivers etc.

By the way this is the XP forum, you might get more help on this on the 98 forum.

Or google how to install windows 98