Source: PlanetHalfLife

A update has been released for Steam addressing some problems people have with starting up the games:

A small number of Half-Life 2 customers have been experiencing problems while playing the game, and we have traced these issues to corrupt Steam cache files. We've released a Steam update to help resolve these issues. If you have been experiencing problems such as crashing during gameplay or startup of Half-Life 2, please restart Steam and then click on one of the links below. Steam will then automatically verify your cache files. The verification process will take roughly five minutes to complete.

To validate Half-Life 2 click here

To validate Counter-Strike: Source click here

To validate Half-Life: Source click here

We're also investigating a solution for the sound skipping problems a small number of users are running into. We'll have a solution for this as soon as possible.

Check the Steam site for more information.