Blizzard says in this post that if you ever participated in the Diablo 3 beta, there is a folder you need to delete if you're going to install the full game:
Quote:
If you have installed the Diablo III beta at any point and intend to install Diablo III, please make sure that all Diablo III beta files are removed from your system prior to installing the full game.
This includes the Battle.net folder, which can be found in the following locations:
[WINDOWS XP]
C:\Documents and Settings\All Users\Application Data\Battle.net
[WINDOWS VISTA/7]
C:\ProgramData\Battle.net
MAC
/Users/Shared/Battle.net
Once this folder and all other beta files are removed, simply run the installer again.
