Summary of the Startup Process

Summary of the Startup Process (x86-based Systems):
CPU initiates motherboard POST routines. Individual adapter POST routines start after the motherboard POST is complete.
The system searches for a boot device according to the boot order setting stored in CMOS. If the boot device is a hard disk, Ntldr starts.
Ntldr switches the CPU to protected mode, starts the file system, and then reads the contents of the Boot.ini file. This information determines the startup options and initial boot menu selections.
Ntdetect.com gathers basic hardware configuration data and passes this information to Ntldr. If more than one hardware profile exists, Windows XP Professional attempts to use the one that is correct for the current configuration.
If the firmware complies with ACPI specifications, Windows XP Professional uses ACPI functionality to enumerate and initialize devices.
Ntldr passes the information collected by Ntdetect.com to Ntoskrnl.exe. Ntoskrnl then loads the kernel, HAL, and registry information. A progress indicator appears near the bottom of the screen.
Networking-related components, such as TCP/IP, load asynchronously with other services and the Begin Logon prompt appears on screen. After a user logs on successfully, Windows XP Professional updates the Last Known Good Configuration information to reflect the current state.
If Windows XP Professional detects new devices, they are assigned system resources. Windows XP Professional extracts the needed driver files from the Driver.cab file or if the driver files are not found, prompts the user to provide them. Device detection occurs asynchronously with the operating system logon process.

Files Processed During Startup:
Boot.ini , Bootsect.dos (multiple-boot systems only), Hal.dll , Ntbootdd.sys , Ntdetect.com , Ntldr , Ntoskrnl.exe , Files in the systemroot\System32\Config folder , Files in thesystemroot\System32\Drivers folder.

(source: MS Technet)