Basically you will need to erase the current partition and create a new one to hold the operating system. be warned that doing that you will lose all your data, I suggest you backup all your personal and criticall data before starting..
Windows 98 comes with a floopy boot disk or a bootable CD, insert that one and boot with it (it will need to boot from the floppy or CD not from the harddisk, if it goes directly to windows 2000 then you need to change your boot order in the bios to Floopy and CDROM first, refer to your manual to how to it)
You will need to run in the command prompt FDISK
In FDISK you will erase the current partition in C and create the new partition in C, after that you will need to format drive C:
reboot, and the harddisk will be ready to hold windows 98
Then proceed with windows 98 install.