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 [color=#bfba50]
restart Steam[/color] 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.
Source:
Steam Powered