A. A. The only requirement for installing Windows NT after Windows 98 is that the system partition (C:) is not FAT32 as Windows NT cannot read FAT32 (at least until version 5.0 of NT which has full FAT32 support).
If the active partition is FAT32 you will need to convert it back to FAT16 or see 'Q. I have Windows 98 installed on C: with FAT32, how do I install NT?'. There are a number of 3rd party applications that can do this, e.g. Partition It from QuarterDeck (http://www.quarterdeck.com). I have never used it but other people have recommended it.
To begin the installation of NT just boot into Windows 98 and run WINNT32.EXE from the Windows NT installation and proceed as normal (select install not upgrade). The Windows NT installation procedure will automatically detect the Windows 98 installation and add it to the NT boot menu.
Do NOT upgrade the system partition to NTFS using the CONVERT.EXE command as Windows 98 will no longer be able to boot.