Probably the NT install was NTFS - if the PC was a company system. That being so, it depends on just how the HD was configured, and where NT was installed; also if there are any NTFS areas in the Extended partition area.
No version of Fdisk will remove NTFS areas from the Extended partition space. Fdisk from DOS 5x/6x/Windows '9x will remove Primary partitions that are NTFS...
The text-book way to remove NT is to use the 3 install disks, and opt to delete partitions when the option comes up during the 'install' sequence.
If you do not have the 3 install disks for NT, with which you can delete NT, then the quick 'n dirty route is to use < delpart.exe >
This is a util that was part of NT3.51 Resource Kit, omitted from NT4 RK, and is available at various places... One is:
http://hercules.lss.ksu.edu/download/MSSTUFF/index.htm
(parent site: http://hercules.lss.ksu.edu)
It's about 30K, fits on a floppy. It will remove just about any partition 'permanently'.
If your NT is NTFS then delpart.exe is probably your easiest option...
If your NT install is FAT16, then just run Fdisk and delete all partitions, repartition as you wish and install '98.
W2K is not for games; alsio it may not have drivers for some of your bit' 'n pieces...; and it is as 'unfriendly' to the unitiated as NT4... , so if you want your family to run games, and other standard items, stay with '98SE. W2K is also an HD hog!
If you use delpart.exe then when all partitions have been removed, run Fdisk etc...