First your going to need the downloader for the installer. Download all of the below US-Installer/Diablo III Setup.exe US-Installer/external_resources.rcc US-Installer/Installer Tome 1. You also connect to an edgesuite server every time you open the launcher.They are as safe as I can possibly make them Let me know how it all goes for you. After you've copied the files over and started up the Blizzard Downloader it should say that it's checking the D3 Installer.
Another way would be to locate your "Diablo-III-8370-en US-Installer" (or equivalent if not in en US) folder and check there's no files and that it's around 7-8 GBish If you are not installing the en US north american english version please take note : You must change all "en US" parts of the links to w/e your locale is.
I wonder how this will affect me whenever I load up the Mists of Pandaria beta.
Will I have to repeat the deleting process each time? it seems the Mo P launcher just reinstalls the /programdata/battlenet file each time I play.
Additional tip for Windows 7 users: Program Data is a hidden folder by default.
You can find it by entering the path into the navigation field, as shown here: press enter).