Welcome to the CH forums.
1. Boot from a Windows 98 boot floppy. If you do not have one, see
http://www.bootdisk.com 2. Run fdisk. Using fdisk, remove all partitions from the drive, and create new ones as needed.
3. At the A:\> prompt type Format C: and hit Enter. The hdd will now be formatted.
4. On completion of Step 3. insert the Win 98 cd into the drive and at the A:\> prompt type R:\setup (where R: is the drive letter of your cdrom, you will have to determine which it is) and hit Enter. Follow the on-screen prompts to install Win 98.
Step 2. can be omitted if you do not want to alter the partitions.
You will need your Win 98 Product Code, drivers as necessary and other software.
Good luck