It sounds like you don't have permission to make the changes. This sometimes happens when the server is not set up correctly, reserving the right to Apache to make all permissions changes instead of the site user.
Try this and see if it works. Create a new text file on your computer and rename it to chmod.php (ignore the warning about changing the file type). Put this code into it and upload it to the root level folder and to the 'admin' folder.
Code:
<?php
chmod ("includes/configure.php", 0777);
?>
Then, using your browser, go to each of those addresses, plus /chmod.php
You won't se anything on your screen, but try the install again. If it now works then you'll still need to contact your host to get them to sort out the permissions issue - as this is a slow and tedious way of changing permissions.
Once you're done, don't leave those files on the server.
Vger