Original sentence: The Windows version of the game is also available for download separately through Valve's content delivery system, Steam and was released as a standalone retail product on April 9, 2008
Edited sentence: The Windows version of the game is also available for download separately through Valve's content delivery system, Steam and was released as a  retail product, (though the buyer still needs to actually operate the game via Steam, continuously on line, and endure the accompanying advertisements) on April 9, 2008