Updating steam platform
Even Source games don’t properly take advantage of content streaming.The menu 3D backgrounds for instance, are a pretty good idea.The Software titles coming to Steam range from creativity to productivity.Many of the launch titles will take advantage of popular Steamworks features, such as easy installation, automatic updating, and the ability to save your work to your personal Steam Cloud space so your files may travel with you.“The 40 million gamers frequenting Steam are interested in more than playing games,” said Mark Richardson at Valve.
However, combined with Steam content streaming, they’re a disaster. Delete the local content for HL2 (or move the out of Steam Apps), then re-install it and try and run the game straight away.
Of course, if you have friends that play that game and they have already download the patch/update, you can exit from Steam and copy those files in the right position of your Steam folders; when Steam restarts it recognizes those new files and you are ready to launch the game.
You can disable the auto-update facility on a per-game basis.
R-click the game in the Library and (off the top of my head) go to Properties and the option is on one of the tabs there.
However, if you are going to be playing what is primarily a multi-player game, you'd be best of just leaving it on otherwise you'll probably not be able to join any games.