The only way to fix that is by replacing the corrupt SAM database in the
\WINDOWS\system32\config folder with one from the \WINDOWS\repair
folder or with one from one of the Snapshot folders in the System Volume
Information folder. It would be preferable to use a recent copy from
one of the Snapshot folders if you can, if you use the SAM file from the
repair folder you will probably lose all the user groups and user
accounts, but their files will still be on the disk.
The only other alternatives are to use your backup software to
completely restore the operating system's System State Data or to do a
*clean* reinstall of the operating system.
You can salvage your files on the hard disk if you need to, post again
if you need more information on how to salvage your files.
http://support.microsoft.com/kb/316751
How to recover from a corrupted registry that prevents Windows XP from
starting
http://support.microsoft.com/kb/307545
(Just do the SAM hive, never mind the others)
Another quick fix to this is to simply delete the SAM file in the
\WINDOWS\system32\config folder. When you reboot the machine you will
have exactly one user account, the built in Administrator account with a
new profile, just like when you boot Windows for the very first time
when you do a clean install. All the other user accounts will be gone,
but their profile folders and their files will still be on the disk in
the Documents and Settings folder. If you wish you can create new
accounts and you should be able to have the new accounts use the old
profiles by assigning them the old paths at the Registry's
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows
NT\CurrentVersion\ProfileList key.
John