NTLDR is an abbreviation for NT Loader, which is a program that resides in a computer hard drive's boot sector. When loaded, NTLDR displays the startup menu of the Microsoft Windows NT operation system. The program can also be loaded from external devices, such as a flash drive or CD-ROM.
NTLDR was used as the boot loader for all Windows NT operating systems up to Windows Server 2003 and Windows XP. For NTLDR to operate, it requires two files, NTLDR and NTDETECT.COM, to reside in the system volume. As of the release of Windows Vista and Windows Server 2008, the NTLDR is split into two parts: the boot manager and the system loader.