The file you downloaded is not an archive, it is the .ipsw file. Your web browser/OS is misidentifying it as a zip archive, and may even be changing the extension. You should NOT open the file with Winrar, but just make sure it is renamed with the .ipsw extension and just use that file. Or try downloading with a different web browser (IE sucks).
Note: Very few people around here are going to recommend Snowbreeze to you. It is really not very good software.