Okay, download it again in another browser making sure you're not using any form of download manager and save the file to your hard disk.
If it still generates the error then override it as follows.
To bypass the corruption test, pass /NCRC on the command line. To do so, go to Start->Run, type cmd and hit Enter. A black console window should open. Drag the installer into the window, hit Space and type /NCRC. Hit Enter again and the installer should start.