WordPress 3.3.1 Installation

Hi, I´m trying to install wordpress and when I try to run http://mydomain.com/wp-admin/install.php, an error message appears:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

The error log says: [20-Aug-2012 13:19:09 UTC] PHP Warning: require_once(/home/segwaytr/public_html/wp-load.php) [function.require-once]: failed to open stream: No such file or directory in /home/segwaytr/public_html/wp-blog-header.php on line 12

I´ve changed permissions, nothing happens
I´ve uploaded a phpinfo.php to see which is my php version. The same error.
.htaccess is empty

Somebody can help me?
Thank you