You can try using lusrmgr.msc from start, run, command box.
The other option is to reinstall Windows XP again which will leave
all files and folders intact. Is good to learn early that playing around with permissions, policies and changing Member of Groups
applies to the Administrator also. There are programs to mess with the Sam file but a reinstall will reset all the changes that
may be stopping you from gaining administrative access to
Here are the instructions for reinstalling Windows XP.