I've been having trouble installing Sins of a Solar Empire -- I receive an error that it cannot find a particular file, but I have verified that the file exists on the disk.  I read one post that suggested installing Impluse and then registering my cd key and downloading the entire game from the website.  I installed Impulse, but I don't know how to register my cd key.  I can see where to buy the game and download it, but I already bought the cd...any advice?