Valve have released an update through Steam which they say resolves problems that users were having previously with corrupt Steam cache files. Restart your Steam client and it will be updated automatically.
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
Download Steam here
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