Computer formatting question

We've got an old HP that's currently running Windows XP. We want to use the restore disks to put it back to the way it was originally with Windows 98 so we can run older computer games on it. If we put the first system recovery disk in (1 of 2) and reboot, it says "Error--The hard disk was not found!" and instructs us to use FDisk. But when we try to use FDisk, it just freezes up. I know the hard drive is there and it works; it's running Windows XP! Any ideas on what we can do to make the FDisk or System Recovery Disks work so we can get this machine back to Windows 98?