NTLDR (abbreviation of NT loader) is the boot loader for all releases of Windows NT operating system from 1993 with the release of Windows NT 3.1 up until Windows XP and Windows Server 2003. From Windows Vista onwards it was replaced by the BOOTMGR bootloader. NTLDR is typically run from the primary storage device, but it can also run from portable storage devices such as a CD-ROM, USB flash drive, or floppy disk. NTLDR can also load a non NT-based operating system given the appropriate boot sector in a file.
NTLDR requires, at the minimum, the following two files to be on the system volume:
An additional important file is boot.ini, which contains boot configuration (if missing, NTLDR will default to \Windows on the first partition of the first hard drive).
NTLDR is launched by the volume boot record of system partition, which is typically written to the disk by the Windows FORMAT or SYS command.