How to Change the Windows XP Administrator Password in Registry
Microsoft Windows XP lets you change the administrator password in the System Registry, which contains Windows XP settings used by your computer while starting up. These settings are also needed to load programs that start automatically during system startup, such as antivirus software. Registry settings are highly sensitive, so be careful when making changes and do not change anything you aren't sure about.
Click on “Start” and then click “Run.”
Type “regedit” (without quotes) in the Run dialog box that opens, then press “Enter.”
Click on the “+” sign next to the “HKEY_LOCAL_MACHINE” folder to expand it on the Registry Editor window that opens. Do the same with the following folders, one after the other: SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion.
Click on the “Winlogon” folder.
Double-click on the “DefaultPassword” key on the right pane of the Registry Editor window.
Type the new administrator password in the “Value data” field of the dialog box that opens and click “OK.”