You will need a boot (floppy) disk to begin with. Get one at, for example, click here
Then restart the pc with that floppy disk.
Then use the fdisk command to clear all that is on the hard disk at the moment - see click here for a guide to using fdisk.
Then format the hard disk.
Then insert your win98 cd and install windows 98.
HAve a look at PC911 for some info about installing windows. click here