Adding www to url brings screen of death

Hi there,

I have recently installed wordpress for a new site I am building. The url automatically appears without the www in front of it. I have created several wordpress sites in the past, and I have always simply added in the www by typing this in to the WordPress Address (URL) and Site Address (URL) on the General Settings dashboard tab.

When I try this with my new site, however, I automatically get the following error message:

Forbidden

You don’t have permission to access /wp-login.php on this server.
Apache/1.3.41 Server at mysite.co.nz Port 80

And the whole site goes down. I can delete the installation and re-install no prob, as I haven’t actually built anything at the site yet, however I need to know how to add www to my website URL (I know that the www is not necessary, but it is for a client who wants it).

Sorry, I know this is probably a trivial and easy solution, but I am at a loss and have been unable to find any solutions after googling / searching forums. As I say, I have been able to do all of this easily for my sites in the past, so can’t think what I could be doing different…