The following is quoted from the Setup.txt file on the Windows 98 CD:
“INSTALLING WINDOWS 98 ON A SYSTEM RUNNING WINDOWS NT
You cannot install Windows 98 over any version of Windows NT, but they can exist together on a single system. However, for compatibility reasons, it is recommended that you install each to a separate hard disk or partition. If Windows NT is already installed, Windows 98 Setup will add itself to the Windows NT boot menu to allow the user to multi-boot between Windows 98 and Windows NT.
If you can no longer boot Windows NT, you should boot from the Windows NT recovery disks and choose the Repair option to restore the Windows NT boot files.
When installing Windows 98 on a system with drives created with Windows NT, you may receive the following error:
‘Setup has detected that your hard disk has a 64K-cluster FAT partition. Because ScanDisk does not work on disks with this cluster size, Setup cannot continue. To complete Setup , you must repartition your hard disk, format the partition with a FAT file system that has a cluster size of 32K or less, and then restart Setup.'
Running Setup with the "/is" parameter (e.g., Setup /is) will bypass ScanDisk and avoid this problem."