Simple solution: create a new file on your home directory, say splash.html, put whatever you would like in this HTML file, then change the default document list for your X-Cart store so that the webserver looks for splash.html before index.html or index.php:
If you have access to the .htaccess file, find the entry for your website and add or edit the DirectoryIndex entry like so:
Code:
DirectoryIndex splash.html index.php index.html index.htm default.htm