Joomla 1.5: How to make a link that points to private area if user is logged on and to login page if not logged in

I try to build a new website with Joomla 1.5

I have a page where I have a link to download. Only if the user is logged in I want him to get there. So I have put the site on "private". Now when the user links this link he gets an error message (404 etc.) because the site is not found. Instead of getting this error message I want to route him to the login page.

The solution could be either to flexible built up the link (depending whether the user is logged in or not) and I do not know how to do this.

The solution could also be to reroute to login page instead of the error page.

Regards,
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