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• You're afraid to take the plunge into a new operating system,
and you want to keep the old one around as a security blanket.
In this unit, it will be explained what you need in order to use dual
booting and how to set up your computer to work with multiple
operating systems. It also be shown you how to remove an
operating system if you find that you no longer need it. Finally,
look at some alternatives to the Windows 2000 boot manager.

Understanding How Dual Booting Works

The boot manager in Windows 2000 Professional supports booting
from the following operating systems:
• MS-DOS
• Windows 95 or Windows 98 (but not both)
• Windows NT (multiple copies)
• Windows 2000 (multiple copies)
When you choose Windows 2000 (or Windows NT, which relies
on the same startup process) from the boot menu, Ntldr executes
Ntdetect.com, which eventually launches Windows 2000 (or Win-
dows NT) from the partition pointed to by the Boot.ini entry you
chose. (Boot.ini specifies the partition by its disk signature and
partition number, which allows you to have multiple installations
of Windows 2000 and Windows NT.)
When you choose MS-DOS, Windows 95, or Windows 98, the
boot manager does something completely different. Instead of
executing Ntdetect.com, Ntldr reads the contents of Bootsect.dos
into memory and performs a warm reboot. The computer then
executes the code in Bootsect.dos as if it were contained in the
master boot.

Understanding the Limitations of Dual
Boot Setups

Making it possible for different generations of operating systems
to coexist on a computer was not an easy task for Microsoft. Over
the years, hardware has changed significantly (remember when the
first hard disks came out, with a capacity of 10 MB!), and
operating systems have evolved to keep pace. This evolution has