The Operating System Windows XP is very complex and uses several files to control it.

• NTLDR
  Controls the operating system boot selection process and hardware detection before the actual Windows XP Kernel is launched. It requires that boot.ini, ntdetect.com, bootsect.dos (if dual booting), and ntbootdd.sys if you are using a non-BIOS SCSI drive.

• boot.ini
  Contains the contents of the boot menu displayed by NTLDR. This file contains the default operating system selection, the timeout period for the selection, and an ARC name or pathname for each listed operating system boot selection.
  When you are on a non-BIOS enabled SCSI drive, scsi() is used instead of multi() in the ARC name.

• ntdetect.com
  Detects the major components of the computer before NTLDR selects a configuration and loads the Kernel.

• bootsect.dos
  Present only on dual-boot systems. NTLDR uses this file when the selected operating system is not Windows XP. Bootsect.dos in turn seeks out the OS-specific operating system loader file, such as io.sys for MS-DOS or os2ldr.exe for OS/2.

  When BOOTSECT.DOS is installed using the DOS-mode setup (for example, a Windows 98 startup disk), the file will still appear, even though it is not really a dual-boot system, meaning there is no COMMAND.com to boot into DOS. There is only one OS entry in the boot.ini file, and the boot selection menu does not appear.

• ntbootdd.sys
  Used only on systems with SCSI drives that do not have on-board BIOS translation enabled. It is a copy of the device driver for your particular SCSI drive.

• hiberfil.sys
  Available if hibernation is enabled on the computer.

Other files can appear in the system partition on x86 dual-boot and multi-boot systems. The addition of these files does not affect the function or capabilities of the required Windows XP boot files in the system partition. Installing multiple operating systems on a single computer with Windows XP Professional (or Home Edition) often requires a specific installation order or manual post-installation configuration changes. Both the Microsoft Windows XP documentation and TechNet include detailed articles on performing multi-boot setups with Windows XP Professional, Windows 2000 Professional, Windows Server 2003, Windows 95, Windows 98, MS-DOS, and OS/2.

based on: The Windows XP Layout; 2003 by Stu Sjouwerman (Que)

also read:

• Microsoft's explanation of troubleshooting the boot process
http://technet.microsoft.com/en-us/library/bb457122.aspx