You were able to play it before, weren't you? And you were using the very same computer?
If so, my best guess is that somehow the game has got corrupted. A hickup during the download or something like that. Try reinstalling the entire Beta game over the existing one to save your settings. If that doesn't work, remove the Beta game from your computer and start from scratch.
Your computer is at the highest end of the entire community, so it's not because lack of power. Unless the culprit is your power supply...