And when all else fails, contact Support and we'll help you ASAP.
Corruption can happen for many reasons, from a connection error somewhere between you and our file hosting provider's servers, to the simple fact of using a wireless connection (a wired connection is generally more robust). One of our users discovered he kept getting corrupt files because his router was old and after he replaced it, the problem was gone. A different case was when a user had a desktop PC with a wireless PCI card - basically it only had a small antenna at the back of the chassis and all the cables etc. were causing a lot of interference. The problems stopped when he purchased and connected an external antenna, which he placed somewhere far away from cables and other electronics. A few Support cases proved that not the users' connections, but their hard drives or RAM were generating read/write errors.
The bottom line is: corruption can happen for any number of reasons. :) Please try the Downloader and, if that doesn't work, you could consider downloading the game on a different connection (a friend's place, for instance, if that's an option). Ultimately send us a message through the Support form and we'll see what we can do. :)