http://support.microsoft.com/?kbid=290260
EFS, Credentials, and Private Keys from Certificates Are Unavailable After a
Password Is Reset
You'll probably have to fix all Accounts you have passworded etc
Windows Home XP
1. As windows is loading up press F8 repeatedly until the Windows advanced
screen comes up.
2. Select safe mode (the version of windows you want to use click XP wait
until it loads the welcome screen.
3. Select Administrator and it will log you on as administrator without a
password (provided a password was not created in this hidden account during
setup).
4. Go to Start - Control Panel - User Accounts.
5. Select your name and click change the password.
Then you should be able to change the password, shut down the computer and
restart in normal mode and use the new password you just created.
****
If you have XP Pro on your computer:-
1. At the log in screen type " ctrl-alt-delete " twice.
2. In the User Account, type Administrator
3. Leave the password blank and press enter.
(Most likely a password wasn't created in this hidden account during setup).
If you decide to set a password it is recommended that you create a
Password Reset Disk by clicking on "Prevent a Forgotten Password" link in
the left pane. The Wizard will guide you through.
http://www2.cajun.net/~theriots/xpsitefolder/win_xp_passwords.htm
Administrator and User Passwords in Windows XP
http://support.microsoft.com/default.aspx?scid=kb;EN-US;321305
How to Log On to Windows XP If You Forget Your Password or Your Password
Expires