did you install any mods? such as custom models and skins? i too have had that happen, when trying to use custom content other than maps. it usually pops up with some 'cant find *.bik'.
to fix it, go to the properties tab, and navigate to local files tab, then hit the "Verify game data" button. that will refresh all the files, download any updates, and such.
another trick to try is, open the properties menu of l4d, go to launch options, and add "-dx9" and save it. then try running the game.