Same thing happened to me recently. I searched the net and found this,
which worked.
Good luck.
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RESET ADMINISTRATORS PASSWORD
1. Place your Windows XP CD in your cd-rom and start your computer (it?s
assumed here that your XP CD is bootable ? as it should be - and that you
have your bios set to boot from CD)
2. Keep your eye on the screen messages for booting to your cd Typically,
it will be ?Press any key to boot from cd?
3. Once you get in, the first screen will indicate that Setup is
inspecting your system and loading files.
4. When you get to the Welcome to Setup screen, press ENTER to Setup
Windows now
5. The Licensing Agreement comes next - Press F8 to accept it.
6. The next screen is the Setup screen which gives you the option to do a
Repair.
It should read something like ?If one of the following Windows XP
installations is damaged, Setup can try to repair it?
Use the up and down arrow keys to select your XP installation (if you only
have one, it should already be selected) and press R to begin the Repair
process.
7. Let the Repair run. Setup will now check your disks and then start
copying files which can take several minutes.
8. Shortly after the Copying Files stage, you will be required to reboot.
(this will happen automatically ? you will see a progress bar stating ?Your
computer will reboot in 15 seconds?
9. During the reboot, do not make the mistake of ?pressing any key? to
boot from the CD again! Setup will resume automatically with the standard
billboard screens and you will notice Installing Windows is highlighted.
10. Keep your eye on the lower left hand side of the screen and when you
see the Installing Devices progress bar, press SHIFT + F10. This is the
security hole! A command console will now open up giving you the potential
for wide access to your system.
11. At the prompt, type NUSRMGR.CPL and press Enter. Voila! You have just
gained graphical access to your User Accounts in the Control Panel.
12. Now simply pick the account you need to change and remove or change
your password as you prefer. If you want to log on without having to enter
your new password, you can type control userpasswords2 at the prompt and
choose to log on without being asked for password. After you?ve made your
changes close the windows, exit the command box and continue on with the
Repair (have your Product key handy).
13. Once the Repair is done, you will be able to log on with your new
password (or without a password if you chose not to use one or if you chose
not to be asked for a password). Your programs and personalized settings
should remain intact.
I tested the above on Windows XP Pro with and without SP1 and also used this
method in a real situation where someone could not remember their password
and it worked like a charm to fix the problem. This security hole allows
access to more than just user accounts. You can also access the Registry and
Policy Editor, for example. And its gui access with mouse control. Of
course, a Product Key will be needed to continue with the Repair after
making the changes, but for anyone intent on gaining access to your system,
this would be no problem.
And in case you are wondering, NO, you cannot cancel install after making
the changes and expect to logon with your new password.
Cancelling will just result in Setup resuming at bootup and your changes
will be lost.