What exactly happens when you try to boot the W98 CD?
There was a time (but I think it was before W98) when you needed a floppy disk to start the install process from CDROM.
It's a very long time since I installed W98 on anything, and memory gets hazy when you reach my age.
Are you trying to get back to W98, or just to install an OS that you can use?
W98 won't like NTFS, so if you've still got the HD formatted that way, expect it to complain. Maybe Mandriva can delete all the partitions on the hard drive and set you up for W98 to partition and format it.
Otherwise, get thee to eBay and buy a cheap hard drive.