The Windows boot process doesn’t begin when you power on your computer or press the reset button. It begins when you install Windows on your computer. At some point during the execution of the Windows Setup program, the system’s primary hard disk is prepared with code that takes part in the boot process. Before we get into what this code does, let’s look at how and where Windows places the code on a disk. Since the early days of MS-DOS, a standard has existed on x86 systems for the way physical hard disks are divided into volumes.
Microsoft operating systems split hard disks into discrete areas known as partitions and use file systems (such as FAT and NTFS) to format each partition into a volume. A hard disk can contain up to four primary partitions. Because this apportioning scheme would limit a disk to four volumes, a special partition type, called an extended partition, further allocates up to four additional partitions within each extended partition. Extended partitions can contain extended partitions, which can contain extended partitions, and so on, making the number of volumes an operating system can place on a disk effectively infinite.
Physical disks are addressed in units known as sectors. A hard disk sector on a BIOS PC is typically 512 bytes. Utilities that prepare hard disks for the definition of volumes, such as the Windows Setup program, write a sector of data called a Master Boot Record (MBR) to the first sector on a hard disk. The MBR includes a fixed amount of space that contains executable instructions (called boot code) and a table (called a partition table) with four entries that define the locations of the primary partitions on the disk. When a BIOS-based computer boots, the first code it executes is called the BIOS, which is encoded into the computer’s ROM. The BIOS selects a boot device, reads that device’s MBR into memory, and transfers control to the code in the MBR.
The MBRs written by Microsoft partitioning tools, such as the one integrated into Windows Setup and the Disk Management MMC snap-in, go through a similar process of reading and transferring control. First, an MBR’s code scans the primary partition table until it locates a partition containing a flag that signals the partition is bootable. When the MBR finds at least one such flag, it reads the first sector from the flagged partition into memory and transfers control to code within the partition. This type of partition is called a system partition, and the first sector of such a partition is called a boot sector. The volume defined for this partition is called the system volume.
Operating systems generally write boot sectors to disk without a user’s involvement. For example, when Windows Setup writes the MBR to a hard disk, it also writes the file system boot code (part of the boot sector) to the first bootable partition of the disk.
Before writing to a partition’s boot sector, Windows Setup ensures that the boot partition is formatted with NTFS, the only supported file system that Windows can boot from, or formats the boot partition (and any other partition) with NTFS. Note that the format of the system partition can be any format that Windows supports (such as FAT32). If partitions are already formatted appropriately, you can instruct Setup to skip this step. After Setup formats the system partition, Setup copies the Boot Manager program (Bootmgr) that Windows uses to the system partition (the system volume).
Another of Setup’s roles is to prepare the Boot Configuration Database (BCD), which on BIOS systems is stored in the \Boot\BCD file on the root directory of the system volume. This file contains options for starting the version of Windows that Setup installs and any preexisting Windows installations. If the BCD already exists, the Setup program simply adds new entries relevant to the new installation.
Source of Information : Microsoft Press Windows Internals 5th Edition