You have to backup NTLDR and NTDETECT.COM because the Windows 2000 versions will not boot XP whereas the XP version will boot W2K and XP. They are hidden system files in the boot drive. Copy them to a blank, formatted floppy, or other location that will be available after you install W2K.

Install Windows 2000 to a different partition than XP is installed in. You now have have W2K's NTLDR and NTDETECT.COM.

You need to repair the Windows XP Boot Loader. Boot into Windows 2000, not Windows XP. Insert the floppy disk that you used earlier (or access the location you used), to restore the XP NTLDR and NTDETECT.COM. Copy them to the root directory of your boot drive.

Now, you'll be able to restart your computer and boot into Win 2K or Win XP.