Assuming some things...
(Like the entire error is actually: "Windows XP could not start because
the following file is missing or corrupt:
\WINDOWS\SYSTEM32\CONFIG\SYSTEM")
... This is likely what you need:
How to recover from a corrupted registry that prevents Windows XP from
starting
http://support.microsoft.com/kb/307545
Just a suggestion - but in the future it would be helpful to you and those
who might attempt to help if you not give your interpretation of the error
messages you receive - but merely the exact message. ;-) In fact - likely
if you wrote down the message word-for-word and did a Google search for
it - you would have found this article in your search.
Now - about this warning in the article (I point it out because most
people have a tendency in these situations to skim past the
non-step-by-step bits too quickly):
"Warning: Do not use the procedure that is described in this article if
your computer has an OEM-installed operating system. The system hive on
OEM installations creates passwords and user accounts that did not exist
previously. If you use the procedure that is described in this article,
you may not be able to log back into the recovery console to restore the
original registry hives."
This may very likely be your case (your computer may have come with
Windows XP pre-installed.) If so - not to worry - you can change the
administrator password *if necessary* to something else by using one of
these methods:
http://www.ubcd4win.com/
http://home.eunet.no/pnordahl/ntpasswd/
Or hints from here:
http://www.petri.co.il/forgot_administrator_password.htm
In the end - if all that looks a bit complicated and/or it does not work -
a repair installation may be just the thing... (that's not hitting the
first R, but the second R) which should keep everything installed intact.
How to Perform a Windows XP Repair Install
http://www.michaelstevenstech.com/XPrepairinstall.htm
How to perform an in-place upgrade (reinstallation) of Windows XP
http://support.microsoft.com/kb/315341
Good Luck!