Okay look at the games section of steam, first right click on the game, then click "delete local content". It will take it out of the "installed" section. Then do this:
Assuming you installed to the default path go to
C:\Program Files\Steam\steamapps\<name of your account>\
Then delete "counter-strike source", and if you have half-life 2 then I would delete that since they share the same engine.
Reinstall the game(s) and see if that works. All of this is assuming you haven't changed your computer since you last played it.