Alright, so I've been slowly downloading Diablo 3 for close to a week, having limited times when I can devote my internet to downloading. Finally today I let it have the entire morning to finish, and when I came home to check on it found that the downloader had closed and that the whole game (or at least approx. 7.6 gigs) was sitting in the installer folder.

Hurray.

I ran the setup and was greeted, as many have been, by the message that I have to insert an installation disc or reinstall from battle.net. Ok, whatever, I have the entire game downloaded, can't take that long right?

Wrong.

I restarted the installer just to make sure it didn't crash or something at 99%, it started checking what I already had downloaded as usual, only at 3.6 gigs it started downloading again. I was confused, as I had seen at least 7 gigs of data in the folder. I opened the folder up again to find that 4 gigs of data that was there minutes before was gone. In my C drive (where I attempted to install the game) I have a folder called Diablo III.temp,
and within that folder is ~1 gig of audio data. So, now it seems like I have to re-download half the game, a process which will take a good 10 hours at least, but I am loathe to even bother. I really don't want to wait all day to download the game, only to have the installer delete half of it again.

Has anyone else encountered this problem? Is there some way to avoid losing so much data if I encounter a problem?

tl;dr: Downloaded entire (or at least 7+ gigs) of game. Ran installer. Installer failed. 4 gigs of data now gone.