Boot off of a clean Windows XP cd ( your bios has to be set to boot from cd rom 1st ) choose Install ( Not > To Repair a Windows XP installation using Recovery Console, press R ) at the first prompt.
It will then search for a previous copy of Windows.
When it finds your copy, it will stop at the next prompt and ask if you want to do a repair. Hit R at this screen.
If it does'nt mention a repair > DO NOT continue.
It will then copy a lot of files and appear to be doing a reinstall ( But its not. All your programs and data are safe, all you will lose, are your Windows updates )
This will allow you to add accounts to the system that will be administrator accounts, it will also fix most problems in XP.