I recently bought the game, but have been able to play it for a while. However, upon registering it on Steam, the single player version doesn't work anymore.
When starting the game, the three intro videos are just white backgrounds (the sound work), and after the videos, when the main menu should appear, I get the message "can't find background image 'materials/console/background01.vtf'" and the game shuts down.
Additionally, starting the game from Steam (instead of the desktop shortcut) doesn't work either - it says that Steam is validating the game, but when that's done, nothing else happens.
Multiplayer works fine.