The first step is for the system BIOS is to test and initialise

1. Power
2. Video cards
3. Hard disks
4. Determine the different devices attached to the PC
5. Test and initialise real memory
6. Plug and play devices

Should any problems occur during the BIOS boot process, the BIOS will issue error codes.
If the BIOS boot completes successfully, it then starts the process of booting the DOS operating system from the first boot device it finds as defined in the BIOS setup process. Control is then given to DOS.

Assuming that the BIOS finds a boot sector on a device, the process of loading the operating system begins. If the operating system is DOS, or any variant of Windows that starts out by booting the equivalent of DOS--which is all of them other than Windows NT or Windows 2000--then a specific operating system load sequence commences, which is normally called the DOS Boot Process. In the case of Windows, additional steps are added to the end of the process after the underlying DOS operating system has loaded.

There are many parallels in the DOS family (DOS/Win9x/ME) boot procedure and the NT family (WinNT/2000/XP) boot procedure. Unfortunately, this scheme does not work with XP.
The MBR is universal whether it is for DOS, Win9x, WinNT, or even for Linux. On the other hand, once the boot up partition is selected the corresponding boot sector (the first sector of the partition) supplies a specific boot code which is either for the DOS family (DOS/Win9x/ME) or the NT family.

In the DOS family, the first file loaded will be C:\IO.SYS which is Windows-version specific. That is, when a particular IO.SYS is loaded, it accepts only the proper version of Windows and fails to boot up with any other version.
The next file looked at is C:\MSDOS.SYS which supplies the location of the Windows system directory. So, once the IO.SYS is loaded, you cannot choose Windows versions --- this is why it is difficult to have multiple-versions of DOS family OS to be loaded.

On the other hand, in the NT family, the first file loaded will be NTLDR which is not version-specific. Therefore, you may select which version within the NT family to load (WinNT4 or Win2000). But if you mix NT versions, make sure that you use the NTLDR module which came from the newer version. That is, Win2000's NTLDR is good for NT4 but NT4's NTLDR cannot load Win2000.