Have you tried Safe Mode? Right after the BIOS post, press F8 or tap every second till you see the Safe Mode options screen, select Safe Mode with Neetworking and press Enter. Go to the winhelponline article and Reset the default security permissions according to instructions.
If you still don't recover the administrator account with default permissions, scan for virus with an online scanner.. if you can install programs, install Malwarebytes Anti-Malware, or download the Hiren's Boot CD or USB and burn to CD/install on USB drive, boot from it, and scan for virus/malware with the included tools..
Reset the Registry and the File Permissions in Windows XP
http://www.winhelponline.com/blog/reset-the-registry-and-the-file-permissions-in-windows-xp/