Thread:Project:Support desk/Cannot edit LocalSettings.php

I have installed Mediawiki 1.18.1. I am using PHP 5.3.9 and MySQL 5.1. Everytime that I refresh my Maain Page, I am propmted to download the LocalSettings.php file which if saved overwrites all the changes I made to the file. If I donot save it, it gives a HTTP 500 error. Please help. Should I change some parameters in the installation?