There are various ways to get rid of NT; one requires the (3) NT boot-disks; another uses a dos/'9x boot-disk; another is the delpart util.
You can either: run Fdisk routine via a '98 boot-disk and delete/remove all partitions. NTFS areas show up as non-DOS; Extended areas have to go before Primary areas; and Fdisk will not remove ntfs areas from an Extended partition location - only from a Primary location...). Or you can go the "kwik n'dirty" way to clear your drive prior to installing '98, i.e. download/use delpart.exe.
It fits easily onto a floppy, or it can be added to '98 boot-disk and run from there. You boot up with a '98 bot-disk and at the a:>\ prompt either change to the delpart floppy or run the delpart util direct from the '98 disk if added to it already. Follow on-screen prompts for using delpart. Afterwards regardless of how you lose NT... you reconfigure drive afresh for '98 and install 98.
Delpart is a freebie; was part of NT3.1 Resource Kit; omitted from subsequent versions... It will remove just about any partition - permanently; can be found at:
http://hercules.lss.ksu.edu/download/MSSTUFF/index.htm
http://www.mesich.com
Useful (how to use Fdisk, install '98 etc....) sites:
http://www.btinternet.com/~robert.bale1/formatinstall.htm
http://www.compguystechweb.com/index.html
To install '98 you either need a computer that allows a CD boot up option (i.e. it can be set in the bios to allow a CD to boot the system as opposed to the normal floppy option), or you use a '98 boot-disk (with CDROM support. If you have a full version CD of '98 it will (should) be bootable, and should come with a boot-floppy too; similarly an Upgrade version is (usually) bootable and should have a boot-floppy included in the 'box'. The boot-floppy approach is probably easier, rather than going into bios settings to change boot-sequence options; easier if not used to going into the innards/holy of holies in a PC bios setup...?
Bootdisk of various 'flavours' can be downloaded at:
http://www.bootdisk.com.
You download the file (it's an image) to a hard-drive; click on it - it (self-) expands to a floppy and you use that to boot, run Fdisk etc., run '98 setup etc... Ensure you get the '98SE (with CDROM drivers) version.
fat16 is the original M$ file format; it allows partition sizes to a max of 2Gig.
fat32 is the another file format; allows partitions in excess of 2Gig... fat32 is a more efficient format for partitions in excess of 540Meg.
Windows '95A uses fat16; '95-OSR2b\c uses fat16 and fat32. '98 uses fat16 and fat32. NT can only use fat16 and ntfs4.
SCSI = Small Computer Systems Interface... It's a system that allows various additional hardware (internal/external) items to be connected in a string (daisy-chained) to one interface card in the PC (a little like current USB, but a totally different system). Devices can include hard-drives, CDROM, scanners etc. SCSI hard-drives allows faster transfers of data to conventional EIDE drives; EIDE drives etc. (and other non-SCSI - and non USB - devices) cannot be 'daisy-chained...' It is pretty much in use with NT systems (servers, multiple-drive systems etc.; was sometimes used in workstations - usually NT but also other windows OS. Many current drives can match performance of SCSI and usually for lower costs. Installing a SCSI device can be a little labour intensive...; not the necessarily the most user-friendly. USB may have displaced it somewhat, particularly with regard to external add-ons? Apple-Mac systems used to use SCS exclusively; I'm not sure if Apple now uses EIDE as an option to, or alongside SCSI?
When you reconfigure (prepare) the drive for '98 I suggest you configure it to have both a Primary partition (for OS, applications, utilities), and an Extended partition (for data). This arrangement allows you to re-install to (even reformat) the Primary partition with out affecting/damaging data in Extended partition (unless one is really careless...); each partition is separate from the other. Putting everything in one partition isn't the wisest/safest way to go; a bit like all one's cash on one bank/stock - if it fails you lose the lot...
For general 'what is help/info/definitions' you may find it useful to know about:
http://whatis.techtarget.com
It does give some useful definitions/info etc.
HTH?
(Need more input - post back; folks here is pretty helpful when you ask for specific help.)