Check the W98 harddisk. If it is UDMA 66 or 100 or 133, it can take the newer 80-wire IDE-cable. If UDMA 33 or lower, it still needs the older 40-wire cable.
If UDMA66+ and your XP-HD is also UDMA66+, check the jumpers on the rear of the disks. The XP must be set as Master or Master-with-slave and connected to the END of the IDE-cable, the W98 as Slave, connected in the middle of the cable.
If this fails, set both to Cable-Select.
If the UDMA-types don't match, attach the W98 to the same cable as your CD/DVD, where CD=master and HD=slave.