No access to admin-site

Hi,
i have problems with logging into the admin site.
The following error occures:

Warning: require_once(C:\XAMPP\htdocs/admin.php): failed to open stream: No such file or directory in C:\XAMPP\htdocs\wp-admin\admin.php on line 12

Fatal error: require_once(): Failed opening required ‘C:\XAMPP\htdocs/admin.php’ (include_path=’.;C:\XAMPP\php\PEAR’) in C:\XAMPP\htdocs\wp-admin\admin.php on line 12

The line 12 of admin.php is:
require_once( dirname(dirname(__FILE__)) . ‘/admin.php’);

What can i do?
regards
Andi