The default DirectoryIndex should already contain an entry for index.cgi. As long as there is not a file in your home directory that appears earlier than index.cgi in DirectoryIndex, and as long as you have   Options +ExecCGI in your .htaccess (or in httpd.conf), you should have a working .CGI as your default home page. I do this both under Apache and IIS (where the configuration steps differ a bit), and it works fine.