A+: NTDETECT.COM plus NTLDR in the boot process

When your PC finishes the Power-On Self Test and the 'bootstrap loader' of its system Basic Input Output System (BIOS) reads the NTLDR file from the root directory of the default Windows drive, it checks BOOT.INI (same location) to show you how many versions of Windows are available, and waits for the specified time for you to choose, before it starts the default version by loading its startup files. 

Then, it has NTDETECT.COM check for installed hardware and puts a list of same in the Registry before it loads the Windows Kernel NTOSKRNL.EXE with its Hardware Abstraction Layer HAL.DLL into memory then passes control to it after getting the device drivers which match your PC's hardware. 

If your PC's Windows drive is SCSI and its host adapter is missing an enabled BIOS onboard, it also has to call NTBOOTDD.SYS, but that's pretty rare since modern PCs almost all use Serial ATA or Parallel ATA drives (aka 'IDE') and many of the newer SCSI adapters have that onboard BIOS.