A hidden, read-only file on the root of the system partition of Microsoft Windows NT that is used during the boot sequence.
The bootsect.dos file is included to support dual-boot installations where another operating system is installed with Windows NT. When the boot loader menu appears, the user selects the operating system he or she wants to boot.
If a non–Windows NT operating system is selected, the ntldr program immediately loads bootsect.dos and turns control of the boot process over to it. Bootsect.dos contains the location of the partition boot sector that existed on the partition prior to the installation of Windows NT.
Thus, bootsect.dos enables the system to locate and load the alternate operating system. Bootsect.dos is not loaded during a normal boot of the Windows NT operating system.