Windows NT startup process

The Windows NT startup process is the process by which Windows NT 4.0, Windows 2000, Windows XP and Windows Server 2003 operating systems initialize. In Windows Vista and later, this process has changed slightly; see Windows Vista startup process for information about what has changed.

The Windows NT installer works very similarly to a regular Windows NT install except that it runs from a CD-ROM. For this boot method to work, the BIOS must be compatible with the El Torito specification. The ISO 9660 file system on the install CD is not fully compatible with the standard. Although it is "Level 1", the file names don't have the file version appended to it. The boot image is of the "no emulation" type, 1 sector long (2048 bytes) and is loaded at segment 0x7c0. It can be extracted from an ISO image by using 7-zip. The ISO image is also not hybridized like ISO images from most Linux distributions and therefore it does not contain any master boot record (MBR) which makes it unable to boot by just copying the image over a block device such as a pen drive.

The installer can also be run from a MS-DOS command prompt so previous versions of Microsoft Windows that are already installed can be upgraded. To run the installer from a MS-DOS based operating system such as Windows 98 or Windows ME, the user must start the system "in DOS mode" and then execute I386/WINNT.EXE on the CD-ROM.

A floppy disk containing MS-DOS can be used to start the installer. Versions of the installer in floppies were also available for sale. From Vista onwards, the installer runs from BOOT.WIM which contains a bootable version of Windows PE. Windows PE is based on the Vista kernel.

On a regular CD-ROM install, the BIOS executes the POST and then searches for a boot descriptor on the CD-ROM. The boot descriptor points to a boot catalog file on the ISO 9660 file system. The BIOS searches for a boot image compatible with the current architecture, loads it into memory and then runs it. The boot image is analogous to the boot sector on a hard drive. The boot image loads SETUPLDR.BIN which is analogous to NTLDR. If this fails for any reason, a message is displayed saying that NTLDR was not found, which may of itself be misleading; moreover, the NTLDR on the CD is never used during the loading phase of the installer. The process also assumes that file versions are unavailable.

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