The intricacies of the inner workings of operating systems are a bit too complicated for most users who just want to see stuff working, without having to go in depth to learn what a software 'mechanism' is all about.
When it comes to things like the boot sequence or having two or more OS's installed, we're already stepping into less known territory. Nonetheless, something quite useful and easy to remember insofar as the multiboot systems are concerned is that they need a good boot loader to function properly.
One of the rather numerous tools of this kind that are mainly targeted at highly experienced users is GRUB4DOS. This program can be considered an evolution of GNU GRUB, carrying several enhancements.
First of all it is important to understand what such a utility actually does. Any boot loader software will come into play when you switch on the power on your computer. This program will initiate the OS or offer a list of installed operating systems to allow the user to choose the one that they want to start.
GRUB4DOS also comes with some extra functions besides the innate ability to boot off through the Windows boot manager, DOS or Linux. More precisely, it allows booting directly from the MBR partition or from a CD and is equipped for BIOS disk emulation, carrying dedicated drivers for ATAPI optical units.
The boot manager or loader of GRUB4DOS can be triggered in a variety of ways. First off, from the DOS prompt or command-line you can run the executable directly. Secondly, it can be integrated in the BOOT.INI file that describes the OS options to be displayed when starting up. Another method would be to incorporate the executable in the CONFIG.SYS file for older editions of Windows (up to Windows NT).
Since it is a highly specialized tool and on top of that one tailored for experienced users only, GRUB4DOS will surely need a bit of getting used to before using it to its full potential.