Boot Sequence in Short:

POST (Power On Self Test): checks memory and other hardware (to check whether they are working properly)

BIOS (Basic Input Output System):  detects the boot device and load and run MBR.

MBR (Master Boot Record): looks at the BOOT SECTOR (the first sector of the active partition). That is where NTLDR is located, The computer loads and initializes the NTLDR file, which is boot loader for Windows XP.

NTLDR (New Technology Loader): read the Boot.ini file, and display the boot menu. If you select Windows XP from the boot menu, NTLDR runs ntdetect.com to gather information about the currently installed hardware.

NTLDR then uses Boot.ini to find the boot partition, the one where Windows XP is installed—and loads the two files that constitute the Windows XP core: Ntoskrnl.exe and Hal.dll. Both files must be located in the %SystemRoot%\ System32 folder

It takes place in following order:

POST --> BIOS --> MBR --> NTLDR --> (boot.ini, ntdetect.com,bootsect.dos,ntbootdd.sys.ntoskrnl)