Mass effect error steam

Hey guys, im desperate, i bought mass effect on steam 2 days ago and still cant get it to work. I downloaded the game and its says 100prosent steam, but when i launch the game i get the message: Couldent load shader file' definitions.usf' Whats wrong?why wont the game work, hope someone can help me

Try this: Click with mouse's right button on the game's name on the games list. Go to Properties, Local Files and choose Verify Integrity of Game Cache