I have the same problem.
I already started over but it doesn’t fix the problem.
I have a the WordPress index.php file in the blog directory.
I tried to run the wp-admin/install.php from IE, I still get the HTML 404 error.
I had no earlier version installed before.
The data in wp-config.php seems to be fine.
I am hosted by GoDaddy.com, if it helps…
Oh and I don’t know if it’s normal, but in my FTP client, I cannot change the permissions.