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Install MS-DOS on a Window NT that uses FAT

Last reviewed: September 2008

On this page:
• About adding MS-DOS 7.10 to Windows NT
• How it works
• Preparation: Dual-Boot MS-DOS on a Windows NT that uses FAT
• Procedure: Installing MS-DOS on a Windows NT that's using FAT
• How to return to your original Windows-only system
• Boot Disks & your own Rescue Boot Disk

About adding MS-DOS to a Windows NT that uses FAT

You can install MS-DOS on a computer with a FAT Windows NT already installed so you can select either MS-DOS or Windows on startup (a dual-boot is created). There will be no loss of data and no commercial utilities are required.

There is no need to create another drive (partition) because both operating systems can safely use the same partition, the C: drive. This page will use the MS-DOS version 7.10 used by Windows 98SE. The hard disk used in testing was a 40 GB Western Digital where Windows NT was installed on a FAT16 partition of just under 2048 MB.

You must not use this page if your current Windows uses NTFS. Use the instructions in Install MS-DOS on a Window NT that uses NTFS - there are substantial differences in the procedures.

How it works

When a Windows NT is installed on a computer that already contains only a MS-DOS installation using FAT, the following occurs automatically during the installation of Windows NT.
1. The existing MS-DOS boot sector is saved to C:\Bootsect.dos.
2. The existing MS-DOS boot sector is over-written by the Windows NT version.
3. A line to call the MS-DOS boot sector in C:\Bootsect.dos is entered into C:\Boot.ini.
A dual-boot of the two operating systems has been created by just installing a newer Microsoft operating system.

To install the older MS-DOS on a computer that already contains a newer Windows installation, we must do the equivalent manually.
1. First create a MS-DOS boot sector and copy MS-DOS boot files to C: with the SYS C: command.
2. NT's 'Inspect Boot Sector' will create C:\Bootsect.dos and recreate the boot sector for Windows NT.
3. In C:\Boot.ini, enter a line that calls the MS-DOS boot sector saved in Bootsect.dos.

The SYS C: command creates the MS-DOS boot sector on C: and also copies, Drvspace.bin, Io.sys, and Msdos.sys to C:.

'Inspect boot sector', run when starting Setup for Windows NT, recognises the MS-DOS sector and copies it into C:\Bootsect.dos. It then writes a Windows NT boot sector on C: (returns the sector to its original state). Unfortunately, it does not alter the Boot.ini file!

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Preparation: Dual-Boot MS-DOS on a Windows NT that uses FAT

  1. It's routine to backup important data first.
  2. You need your Windows NT installation CD or Setup floppies.
    If already dual-booted, you need the CD for the most recent version of Windows.
    You also need the Administrator password if one was set - it's rarely set in home computers.
  3. You need a Windows 98/98SE Startup Disk. Get one at

    If you don't have a floppy drive, you can use a bootable CD created from an ISO or IMA image of MS-DOS using a utility like CDBurnerXP (free) or Nero. There's plenty of MS-DOS images available free on the Web.

    If using an ordinary MS-DOS boot floppy/CD, you must ensure it contains the file.

    Booting MS-DOS from a USB flash drive is still a hit-or-miss affair. However, you might like to Google with 'boot usb flash pen stick drive' or have a look at How To Boot From A USB Flash Drive at
  4. Create a C:\dos71 as a storage folder for DOS files and remember, later on, to enter that location in Autoexec.bat's PATH.
  5. This might be a good time to run Disk Defragmenter, and run Chkdsk /F from a Command Prompt. Allow plenty of time!

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Procedure: Installing MS-DOS on a Windows NT that's using FAT

Reboots are important. Please follow on-screen instructions.

  1. Prepare (read above)

    First make a MS-DOS boot sector on C:
  2. Boot from the Win98/SE boot floppy/CD. Type:
        SYS C:
        Install your full MS-DOS now. Use the C:\dos71 folder except for boot files.
    Reboot (remove floppy). Only MS-DOS boots at this stage.

    Recreate the Windows NT boot sector on C:
  3. Boot from your Windows installation CD or Setup floppies.
        Press R to 'To repair a damaged Windows NT . . .'
        Use up/down arrow keys and [Enter] to remove the X for all options except Inspect boot sector
        ('Inspect boot sector' is the only option we want to execute).
        When ready, highlight Continue (perform selected tasks) and press [Enter].
        When the NT has been located, press [Enter] for 'To repair the Windows . . .'
    Reboot (remove CD/floppy).
    Let Chkdsk finish. Only Windows NT boots at this stage.

    Finally, edit Boot.ini so MS-DOS can be selected during startup
  4. Edit Boot.ini (it's a Read-only, Hidden, file in root of C:) - you can read How to Edit Boot.ini
        Add the following line to the end of the [operating systems] section
        C:\BOOTSECT.DOS=" MS-DOS 7.10 "
        and, optionally, change the timeout under [boot loader] to timeout=10 (that's 10 seconds).
        Save Boot.ini back to C:

Finished! You now have a boot menu containing both operating systems.

If you did not install all the MS-DOS files earlier, you must now copy them to the hard disk. Actually installing MS-DOS at this stage would rewrite the boot sector just like SYS C: does!

* You must recreate and copy Bootsect.dos again if the hard disk geometry is altered again.

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How to return to your original Windows-only system

Remove the line C:\bootsect.dos=" MS-DOS 7.10 " from the Boot.ini file.
Reboot! Windows boots automatically without the appearance of a boot loader screen.

Delete the DOS71 and Bootsbck folders, and any other folders created for use with MS-DOS only.

To fully complete removal, delete the unrequired boot files on the root of the C: drive:
Autoexec.bat, Bootsect.dos,, Config.sys, Drvspace.bin, Io.sys, and Msdos.sys

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NEVER dual-boot without your personal safety net - a BOOT DISK for XP/2K/NT.
If stuck, use a Win9x Startup Disk (CD support), and run WINNT.EXE from I386 folder on CD.

If you don't have a floppy drive, you should investigate how to create a bootable CD from a ISO floppy image.

RESCUE BOOT DISK for when XP, 2K, NT will not boot - ESSENTIAL, 1 floppy
   Create a bootable floppy to get XP/2K/NT running even if the boot record,
   or boot files, are ever a problem. Write-protect and keep it safe.
      Format a floppy with that XP, 2K, or NT. It must be a full XP/2K/NT format.
      Alter file Attributes (Attrib -r -s -h) of these files in root of C: (PC system partition)
      Boot.ini, NTLDR,, and Bootsect.dos & Ntbootdd.sys (if present)
      (plus Arcldr.exe & Arcsetup.exe - for Windows 2K) and copy them to the floppy.
      Write-protect the floppy. Then restore original Attributes to the files on C:.
      Read the Rescue Boot Disk page for fuller details, plus a much improved Rescue disk.
Use the CD: If you have a Bootable installation CD you should check if your BIOS
   supports booting from it. This is hugely convenient, but still make the floppy.
ERD XP, 2K, NT: Emergency Repair Disk - repair key Registry entries and partition geometry
2K/NT Setup Boot Disks: (4 floppies for 2K, or 3 for NT)
   Use WINNT32 and MAKEBT32 from 2K/NT (\Boot disk folder on CD).
   Use WINNT and MAKEBOOT from a non-2K/NT system (such as Win9x).
XP Setup Boot Disks: (6 floppies)
   Read the Microsoft page How to obtain Windows XP Setup boot disks.
   It provides free downloads that create setup boot floppies for all versions of Windows XP.
   Each download is specific to each XP version (Home/Pro; original/SP1/SP2).
Win9x/Me Startup Disk:
   (95+) From a Windows: Control Panel > Add/Remove Programs > Startup Disk tab.
   (98+) From true MS-DOS: Go to the Command folder in Windows, and type Bootdisk.
   Windows 95 Startup Disks do not have CD support (add your CD-ROM driver).
   You can download free Win9x/Me boot disks from (IDE CDrom Drivers Included).

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Please remember that you alone are responsible for the consequences of any changes you make to your computer hardware or software.

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