Reinstall the Source SDK installation

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If all else fails, it may be necessary to re-install the SDK tools. It is advisable that you make a copy of your entire "..\SteamApps\username\sourcesdk" directory before following this procedure. After you've reinstalled the SDK, you have a backup in case there is some content in your sourcesdk directory that you may wish to restore.

Note:Your GameConfig.txt file will be lost in this process, meaning any game configurations for custom MODs you have created will be lost as well. If you wish to retain these configurations, please backup the GameConfig.txt and follow the instructions in Reset the Source SDK Game Configuration. You can ignore this step if you have no custom MODs installed.

1. Shutdown Steam by choosing Exit from the Steam task bar icon menu.
2. Delete the "<Steam Install Directory>\SteamApps\username\sourcesdk" directory, where the SDK resides.
3. Delete the sourcesdk.gcf file from the "SteamApps" directory.
4. Restart Steam and launch the SDK application.

Steam will automatically re-install all the necessary files.