waffulz,
Try verifying your game cache folder.
In the Steam games menu, right click on the game you want to play, click Properties, go to the Local Files tab, and click on Verify Integrity of Game Cache. It will display a loading bar, which takes from a few minutes to around a half hour. If it detects that the Game Cache (The game files) are bad, it starts to download them again. If it detects that everything is OK, it will give you s success message.
If it still doesn't work after the integrity check, try deleting the game files (In the Steam/SteamApps directory). For Source games (Like Half-Life 2, Portal, TF2, etc.) delete the .gcf files with their names on them (Not to be confused with NCF files), then verify the game cache again.
If everything fails, try reinstalling Steam, restarting your computer, or running it as an administrator by right-clicking the Steam shortcut and clicking Run as administrator. Take note that even in the Admin accounts you need to run it as administrator.
