FAT16, 32, or NTFS formatted?
For FAT16/32, download a clean W98SE disk from
www.BootDisk.com and try
FORMAT C: /MBR
if I recall correctly.
For NTFS they might have rescue diskettes, or I think you can make them using the NT4 CD.
This all assumes you're trying to preserve the existing Nt4 install rather than doing a clean install of another OS.