Ntldr, short for "NT Loader," is a crucial component of the Windows operating system, specifically for versions prior to Windows Vista. It is responsible for booting the operating system by loading the Windows kernel and essential system files into memory. When a computer starts, Ntldr reads the boot configuration and determines which operating system to load if multiple options are available. In addition to its primary function, Ntldr also provides a user interface for selecting different operating systems and can manage booting from various partitions. If Ntldr is missing or corrupted, the computer may fail to start, leading to error messages and requiring troubleshooting or repair.