downloading large files through a web browser can result in screwed up files you can either try again on a more reliable internet connection make sure you computer doesn't go to sleep and cross your fingers, find a non web-browser way to download it, or use the torrent from the gamershell mirror(what i used).
also 7zip maybe worth a look if windows just doesn't like your zip file
Another way is our "WCS Setup ZIP Check & UnZip" tool, see link in my sig. It is suitable for the big Zip Archive. The tool can both: checking the zip archive with MD5 and unpacking if the check was successfully.
It's on the german forum. But simply download the first attachment. The tool has english requesters, so it's easy to understand.
I'm not sure about all the download sites and how they are handling the files for downloading; however, I have found if I can't get a file to download correctly using Internet Explorer, it will usually work using Firefox...that's about the only thing I use Firefox for. And as mentioned earlier, 7-Zip is a great tool for unzipping files.
MB: GigaByte GA-X58a-UD3R w Intel X58 Chipset
CPU: Intel Core I7-980x Extreme Edition 3.33GHz
GPU: ATI Radeon HD 5770 1Gb GDDR5 16X PCIe
RAM:12GB DDR3/1333MHz Corsair Triple Channel
Sound: Creative Labs X-FI XtremeGamer 24-BIT PCI