Verify integrity of game cache
Load Steam
From the My Games tab, right-click on the game and select Properties from the menu.
Select the Local files tab and click the Verify integrity of game cache... button.
Steam will verify the game cache files (GCF's) - this process may take several minutes.
Once the process is completed, the Check Window will automatically exit.
Re-extract the Game Files
Once you have verified the integrity of your game cache files, you can make sure that your extracted game files are correct by navigating to your Steam user directory: ..\Steam\steamapps\[USER NAME]\ and either renaming or deleting your game folder.
Note: If you are playing a Singleplayer game and wish to keep your saved games, rename the game directory.
The next time you run the game, it will re-extract the files from your game cache files. If you wish to restore your saved games, copy the SAVE folder from your old game directory to your new one.
support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/std_adp.php?p_faqid=282
If that doesn't apply then here is how to perform a proper, clean reinstall of Steam:
Write down your Steam Account name and password - you will need them to log back into Steam once you have reinstalled
Logout of Steam if you are able to do so
Exit Steam
Move SteamApps out of the Steam folder. This is the folder that holds your games so you want to keep it!
If you are having problems with a specific game, delete it's folder from SteamApps/<account> - Uninstall via Add/Remove programs, if you can
Restart if prompted
Delete Steam folder if it still exists
Hit windows key + r ( run at the start menu ) and type regedit
Delete HKEY_CURRENT_USER\Software\Valve and HKEY_LOCAL_MACHINE\SOFTWARE\Valve (or HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Valve on 64-bit Windows)
Restart
Download and install a fresh copy of the Steam installer from
www.steampowered.com/download/SteamInstall.msi Run the installer
Exit Steam before you first login
Move your SteamApps folder back
Run Steam.
Retrieved from "http://developer.valvesoftware.com/wiki/Cleanly_reinstalling_Steam"
