In the Bios set "boot options" to boot from CD Rom;
Insert your Win98 into CD ROM and install (following instructions).
If you do not have this option, then set it to boot from floppy. You will need the Win 98 boot disk, or something similar with generic CD ROM drivers. Once the floppy boots, it will give you the drive letter of the CD ROm eg. D:
Type d: <enter>
Type setup <enter>
and install Win 98 (following the instructions).
It is best to install the mobo CD Rom first because they may have updated USB, AGP etc drivers, then install, sound etc.
Always reboot after installing each one, so if any problems comes up you know which driver cause the problem.