Restricting access to web site

[Dang2012]

We have our intranet web site hosted on a computer running Windows XP on our local network. The web site is created using XAMPP 1.7.1 and Joomla 1.5. Right now all internal users can access the web site.
For some special reasons, one external user needs to be able to view our Intranet site. What do I need to do to allow access to our intranet web site only to this one particular external user?
Please advise me if this question should be asked in Joomla forum.

Thanks in advance for your assitance.