multiple blogs help

i’m still an amateur so i need a little help. 😛

okay, i got the idea of installing multiple blogs in a single domain by installing them separately but how about adding a new blog on an existing blog? I mean, the previous installation was directly uploaded in the public_html?

what i mean is this.
*Existing installation is this:

http://domain.com – current path of the wordpress installation
public_html>wp_admin, wp_content and etc

what i want to do is to add another wordpress separately for a portfolio ‘http://portfolio.domain.com’

How can I do that?
Thanks in advance.