When you setup Windows NT, the BIOS parameter block of the previous operating system on <Drive C:>, sector 0, is written into the Bootsect.dos file.
To recreate it:
1. Boot an MS-DOS floppy and run the SYS C: command (or follow the procedure indicated for your previous operating system).
2. Boot the Windows NT setup floppy disk 1 and run a repair, selecting only:
Verify boot files on your C: drive
3. Once the repair is complete, you should be able to boot your previous operating system.
NOTE: If you misplaced your Windows NT setup floppies,
run WINNT.EXE /OX from the CD-ROM or
run WINNT32.EXE /OX from Windows NT.