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.