It could very well be one of the technical issues detailed in the thread I linked to in my last post. Find the exact problem by running the game again, and seeing what problem the computer mentions then check the thread out, and look in the FAQ thread linked there. If it's not there post in the technical issues forum, also linked in the thread I've linked.
Originally Posted by sgrubb
buy real copy instead of piratebay
Don't make assumptions like that, often problems like this are caused by computer problems, software problems or currupted data/DVD. The OP has already mentioned Steam so he's probably bought it legally.