Left 4 Dead 2 Beta

Cardboard RPG-7 Manufacturer 28 feb. 2014 a las 13:17
Compatability Help
i bought L4D2 about a month and a half ago. it came with the game itself and it came with the beta. every single time i try to play the actual game it boots up and there is sound and all but my screen goes black with "unsupported" written on the screen. But, i can play the beta version with no problems whatsoever. i want to play the real game because of their are many more mods on the full game. im playing on windows 8.1. can anyone give me some advice on how to get the game to work right?

tl;dr: how do i play L4D2 on win. 8.1?