If you cannot log on to Windows by using a particular user account, but you can log on to another account that has administrative credentials, follow these steps on how to do the trick:
Log on to Windows by using an administrator account that has a password that you remember. You may need to start WinXP in safe mode.
Click Start, and then click Run.
In the Open box, type “control userpasswords2″, and then click OK.
Click the user account that you forgot the password for, and then click Reset Password.
Type a new password in both the New password and the Confirm new password boxes, and then click OK.