If you had installed WordPress on your server and you deleted it sometimes you may get this error after trying to install it automaticaly again.
You cannot install more than one script in the root directory of a domain
That usually happens if you deleted older WP partly by hand and partly automaticaly with Fantastico, Quick Install or some other Cpanel script. At Install WordPress 2/3 part, the installation just can not be completed.
Solution for resolving that issue is the following:
Log on server by FTP (or Cpanel File Manager).
Open .fantasticodata folder in your root directory.
There find the file installed_in_root.php
Download it and open in some txt editor (Notepad ++ is great for that) or just open in file editor online if you are using Cpanel.
The file shoud have this:
$installed_in_root["yourdomain.com"] = 1;
Just change that 1 to 0 so you get.
$installed_in_root["yourdomain.com"] = 0;
Save that and upload on server.
After that, you should be able to install new WP normal.