Try the following
Please start with these steps when you are ready to troubleshoot the cause of the crashes you are experiencing.

1. Check the Minimum System Requirements If you have never played the game before on your system, it is crucial to double-check the system requirements to determine if your system can run the game. The minimum system requirements for each game can be found on the game's page in the Steam Storefront.
   Check the DirectX and Pixel Shader Requirements for Games on Steam topic to make sure that your system meets the minimum requirements for the game you are attempting to play. Many newer games such as Team Fortress 2 and Portal require greater system resources than Half-Life 2.
   
2. Verify Your Game Cache Files Please see the Verifying GCF Cache Files topic for instructions to ensure game cache file corruption is not the source of the crash.
   
3. Rule out Background Applications and Anti-Virus/Security Software Check the Programs Which Interfere with Steam, and Spyware, Adware, and Viruses Interfering with Steam topics to ensure the crash which you have encountered is not related to malicious software or a specific program which is known to cause issues with Steam.
   Certain Anti-Virus applications such as AVG 7.5, Nod32 and Norton have been confirmed to cause instability with Steam and Source Engine games. This software must be fully uninstalled from your system until the Anti-Virus software developers can resolve these issues. If you are not comfortable with using alternative Anti-Virus protection, then we recommend contacting your Anti-Virus developer for more information.
   
4. Do Not Use Two Sound Devices Simultaneously If you have installed a PCI or USB sound card on your system (such as a Creative X-fi or Audigy sound card), please ensure that your motherboard's onboard sound card is disabled in the system BIOS. Having both installed may result in system instability - even if the secondary device is disabled in the Windows device manager. Please consult your motherboard's manual for instructions on editing BIOS settings.
   
5. Routers and Firewalls Can Also Cause Crashes Check the Router and Router Firmware Issues topic to make sure that your router is properly configured and that you are running the best possible firmware version available. If your router is not configured correctly, it can cause crashes both in Steam and in games.
   Firewalls can also lead to crashes depending on the way they are configured. Please follow the Using a Firewall with Steam topic to make sure your Firewall is properly configured.
   
6. Force the Game Settings Back to the "Factory Defaults" Those custom autoexec.cfg files and graphics tweak guides you followed promised all kinds of improvements, but if all you ended up with are crashes then it is time to restore the original game settings for stability. In order to do this, make sure you are not running any custom autoexec.cfg files when the game starts up and use the -autoconfig launch option. To set this option, follow these steps:
   
   • Right-click on the game you're attempting to play
   • Select "Properties"
   • Click "Set Launch Options", found under the "General" tab
   • Type in: -autoconfig
   • Click "Ok"
   • Launch the game
7. Mss32.dll (Miles Sound System) Other PC games may install the Mss32.dll file in your C:\Windows\System32 directory. This can lead to the game crashing because the Mss32.dll file in your Windows folder is conflicting with the file located in the game's directory. Please see the The MSS DLL is incorrectly installed in the Windows System directory topic for instructions on how to remove the file from your System32 directory.