copy index.php into your main folder
edit index.php to point to your wordpress folder
under Settings: General, change Blog URL to your main url.
done
example:
wordpress is installed in public_html/wordpress/
copy public_html/wordpress/index.php into public_html/index.php
edit public_html/index.php and you'll find this line
Code:
require('./wp-blog-header.php');
change that to
Code:
require('./wordpress/wp-blog-header.php');