IF you have a bootable Win98 CD just start the macchine with that in there. Make sure the machinne is set in BIOS to boot from CDROM first.
If not a bootable CD ROM, or if you want to do it the other day, bo to
www.bootdisk.com for a free boot disk for any operating system.
In any case, you will want to delete all partitions, recreate a partion and format the drive prior to reinstall. Post back if you don't understand this.
Make sure and back up any data first, if needed. You may want to write down what devices are identified in Device Manager before blanking everything out.