Windows XP Administrator Priviledges/Internet Explorer

Internet Explorer will not let me modify the Internet
Options under Tools menu as it claims I am not an
administrator despite being the only user on the system,
listed with Administrator priviledges.

Is there a registry setting that can be changed
to 'renew'/give me back my Administrator priviledges?

I can install software happily and have 99.5% use of
the 'power features' of my system.

regards,
John