http://support.microsoft.com/kb/321305

    Log on as an administrator Restart the computer Press              .
    CTRL+ALT+DELETE two times, and then type the name of the           .
    administrator user account                                         .

        Note If you do not know any other administrator user account,
        try to type Adminstrator in the User name box.  In the Password
        box, type the administrator password, and then click OK.

    Note If you do not know the administrator password, leave it blank,
    and then click OK. Reset the password. Click Start, and then click
    Run. Type control userpasswords2, and then click OK. On the Users
    tab, click the name of the user account that you want to reset the
    password for, and then click Reset Password. Type a new password
    in the New password box, and then type the password again in the
    Confirm new password box, and then click OK.

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http://www.techbite.in/2009/01/reset-xp-password.html

Restart the computer, and then try to log on to Windows XP again. On the
welcome screen press CTRL+ALT+DEL twice. Type username as Adminstrator
and leave the password blank and press enter. If you're lucky then
you'll enter the system. Lucky in the sense that, if the system owner
didn't enter an adminstrator password while installing windows then
you're lucky.