Seems like a lot are having issues with installing Diablo 3.
I've had no end of trouble with mainly "An unexpected error occurred while trying to install. Please try again or contact Customer Suppport.".
After trying numerous suggestions it looks like I may have cracked it.
I re-downloaded the installer again (even though I already had the newest one), and ran it. It downloaded some stuff but then I still had an issue where it said I did not have the correct OS. I restarted my PC, reran the installer and this time it seems to be installing correctly - at 77%. So looking good.
Hopefully the rest of you have had fewer troubles and have been enjoying the game