When you repaired the Windows 2000 installation it replaced the Windows
XP version of the files ntldr and NTDETECT.COM with its (Windows 2000)
versions.  The Windows 2000 version of these files cannot boot Windows
XP, the Windows XP version of the files can boot the earlier Windows
2000/NT versions.  The fix to that problem is to replace these files
with the XP versions after you do the Windows 2000 repair.  You can copy
the files from the XP cd or you can save them on a boot floppy and then
copy them back to the root of the System drive.  The boot floppy with
the XP versions can also be used to boot the XP installation.  To create
a boot floppy format a diskette with Windows NT/2000/XP and then copy
the files boot.ini, ntldr and NTDETECT.COM onto the floppy.  The
diskette *must* be formated with an NT type computer, do not format the
diskette with a Windows 9x machine, if you do the diskette will fail to
boot the NT/2000/XP installation.  To boot with the diskette set the
boot order in the BIOS to diskette first then insert the boot floppy in
the machine and boot the computer.

As for the Systemced missing or corrupt on Windows 2000 that is usually
caused by the System hive being corrupt or too large.  You can attempt
to repair that by restoring the Windows 2000 System hive in the
system32\config folder with a backup one in the
%systemroot%\repair\regback folder.  That can be done while you are
booted to the Windows XP installation.

Error Message: Windows Could Not Start Because the Following File Is
Missing or Corrupt: \Winnt\System32\Config\Systemced
http://support.microsoft.com/kb/269075

http://search.microsoft.com/results.aspx?mkt=en-US&setlang=en-US&q=systemced

John