Global Previous/Next Links – How?

It has been over a week since I posted this request, so I think I’m safe in reiterating it again, for someone who can help me to take it one step further.
I’m trying to get the previous/next tag (<?php posts_nav_link(‘separator’,’previous’,’next’); ?>) to display a chronological navigation. Using this tag in index.php, the “next” link is looping back to the first post in the weblog. BTW, my weblog is set at “1 posts paged”.
I would like the “next” link to not do that. In fact, I would not like it to show on the index.php page, but show on all other pages. I would, however, like a “previous” link on the index.php page that goes back chronologically. So, I guess there are two requests here: get rid of the “next” link and add a “previous” link to the index.php page.
In the aforementioned thread, it was suggested that I do something with an if else combination:

Something like :
(in your index.php file)
If current page is “index.php” (and nothing else after that address…)
then echo <?php posts_nav_link('seperator','previous',''); ?>
else echo <?php posts_nav_link('seperator','previous','next'); ?>

I have no idea how to do this. So, I am asking for some help, please.