Win XP Password Changed - user account

Hi
Someone somehow has got into my pc & changed the user passwords.

I am running norton antivirus & have all security settings on & have been running updates but still happened.

I looked on web & someone mentioned if reboot using sys discs & press shift f10 when device manager reloading this shortcuts to the password manager but got "doesnt recognise password" & thus no cigar.

I have searched the forums for this issue but not found exact same.

If there is an existing thread sorry can you fwd to me.

Otherwise appreciate help.

Thanks,
M