Maybe Because you are using Windows 7. This game was made in 2004 so there is no Windows 7 yet so I have one thing I suggest you to do:
1. Go to the Game directory (Counter-strike Source Folder)
2. Right click on Run_css.exe (CSS Launcher) then, properties (I'm using XP I dont know in 7) and Compatibility then 'Run this program in compatibility mode for:'
3. Select Windows XP ( or if there is no xp select other).
Let us see if that Works!
That is the Common problems in Old Games thats running in New OS.