See here for step by step instructions:
http://support.microsoft.com/default.aspx?scid=KB;en-us;306214&
Can't Log On to Windows XP?
If that’s your only problem, then you probably have nothing to worry about. As long as you have your Windows XP CD, you can get back into your system using a simple but effective method made possible by a little known access hole in Windows XP.
This method is easy enough for newbies to follow – it doesn’t require using the Recovery Console or any complicated commands. And it’s free - I mention that because you can pay two hundred dollars for an emergency download of Winternals ERD with Locksmith which is a utility for unlocking lost Windows passwords. See here http://www.winternals.com/products/repairandrecovery/locksmith.asp
ERD is an excellent multi purpose product, but you should know it is not a necessary one if you have a healthy system and your sole problem is the inability to logon to Windows due to a forgotten password. Not necessary because you can easily change or wipe out your Administrator password for free during a Windows XP Repair. Here’s how with a step-by-step description of the initial Repair process included for newbie’s.
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.
Ok, now that your logon problem is fixed, you should make a point to prevent it from ever happening again by creating a Password Reset Disk. This is a floppy disk you can use in the event you ever forget your log on password. It allows you to set a new password.
Here's how to create one if your computer is NOT on a domain:
- Go to the Control Panel and open up User Accounts.
- Choose your account (under Pick An Account to Change) and under Related Tasks, click "Prevent a forgotten password".
- This will initiate a wizard.
- Click Next and then insert a blank formatted floppy disk into your A: drive.
- Click Next and enter your logon password in the password box.
- Click Next to begin the creation of your Password disk.
- Once completed, label and save the disk to a safe place
How to Log on to your PC Using Your Password Reset Disk
Start your computer and at the logon screen, click your user name and leave the password box blank or just type in anything. This will bring up a Logon Failure box and you will then see the option to use your Password Reset disk to create a new password. Click it which will initiate the Password Reset wizard. Insert your password reset disk into your floppy drive and follow the wizard which will let you choose a new password to use for your account.
Note: If your computer is part of a domain, the procedure for creating a password disk is different.
