Call of duty:world at war not starting?

i bought call of duty: world at war off steam. it took 6 hours to download and i was all excited to play it. but when i tried to launch it, it just says "preparing to launch call of duty: world at war" and the box just closes and nothing launches. i tried to use punk buster to update it but that did nothing. i own a microsoft windows 7 32-bit computer. any help please?