Fatal error in comments.php on installation

Hi!
I’ve been trying to install WP 2.6 for over seven times now. But everytime it doesn’t work. I keep getting the same error:
Fatal error: Call to undefined function is_admin() in /storage/mijndomein/users/120064/public/sites/www.kinderdagverblijfdedassenburcht.nl/wp-admin/includes/comment.php on line 110

I’ve tried downloading the .zip file and the tar.gz file but I keep getting the same error, even after seven installations. On my own weblog it worked like a charm (with the same provider) but it keeps on going wrong here.
I’ve removed the config-file now, because otherwise all the details are out and open. But a copy of it you can see here: https://i155.photobucket.com/albums/s303/dreamflower_kiaora/helpp.jpg