If you are one of two million people who played the Diablo 3 beta, you must totally uninstall the beta — and delete the Battle.net folders — before installing the full game.
The Battle.net folder can be found in the following locations:
C:\Documents and Settings\All Users\Application Data\Battle.net
Once this folder and all other beta files are removed, run the installer. Be sure you are logged on an administrator account when installing the game.
Run the Diablo III Installer in Administrator Mode (Windows Vista, 7, 8)
In addition to being logged on an Administrative account, you may also need to run the installer with the Run as Administrator command.
* For the digital version of Diablo III, right-click the Diablo III installer icon and select Run as administrator.
* For the DVD version, click Start and Computer, then right-click the DVD and choose Open. Right-click on the Installer listed as “application” and select Run as Administrator.
Battle.net Update Agent (Agent.exe) Issues
Issues with the Agent helper program may prevent Diablo III from installing or updating properly. If you notice your installation hanging, or if you receive errors specific to Agent, try the following steps:
1. Stop the installation process if it’s still running.
2. Wait for at least one minute. Ensure that Agent.exe no longer appears in the Processes tab of your Task Manager.
3. Restart the installation.
If that does not resolve the issue, delete the C:\ProgramData\Battle.net folder and try the installation again. The launcher will automatically rebuild this folder.