Is Steam required for Half-Life and other Gold Source games

http://support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/std_adp.php?p_faqid=477

In the past, Steam would recognize and make use of files installed from CD for Half-Life and Counter-Strike. As many of the game files have been updated from the time these CD's were published, Steam no longer supports this feature (the game content will need to be downloaded again anyway).

To save time reinstalling in the future, it is recommended that you use Steam's backup feature. Please see the Using the Steam Backup Feature topic if you would like instructions for backing up your games.