Previous and next link

I added the following code to the index.php page: <div class=”navigation”><p><?php posts_nav_link(); ?></p></div> but there is something wrong with the link that appears: http://www.english-channel.org/index.php/Index.php/page/2
I added the same code (<div class=”navigation”><p><?php posts_nav_link(); ?></p></div>) to archive.php and it works fine.
To work fine, the link should be http://www.english-channel.org/index.php/page/2 when I click on “Next page”.
Could anybody please tell me how to get rid of the “Index.php” part?

Thanks for your help!