Before you begin make a backup copy of the system files ntldr and NTDETECT.COM , these files are in the root of the System drive, (usually C:). When Windows 2000 installs itself it will replace the Windows XP version of these system files with its own earlier version. The Windows 2000 version of the files cannot boot the newer Windows XP operating system, the Windows XP version can boot the older Windows 2000. After Windows 2000 is installed you can just copy the XP verision of the files back to their C:\ location, you will then be able to boot both operating systems.
To copy the Windows 2000 setup files and launch the setup program use the /makelocalsource switch, launch the command at a Command Prompt:
X:\i386\winnt32 /makelocalsource
where X is the drive letter of the CD-ROM drive, the setup program will then prompt you through the rest. Pay attention to the location of the Setup files in the default setup options (Advanced Options). After the files are copied the computer will reboot and you will be able to cleanly install Windows 2000 on the partition of your choice. The setup program will automatically add the Windows 2000 installation to the boot.ini file and you will be presented with a dual boot menu when the computer boots up.
To get help on the winnt32.exe command use the /? switch:
X:\i386\winnt32 /?
John