See if you have the Windows Secondary Logon service enabled. The game requires it and I know a lot of people disable it in hopes of making their computer more secure. It's enabled by default by Microsoft.
To check, go to Control Panel > Administrative Tools > Services and scroll down to Secondary Logon. right click it and select properties. Check it's startup status and change it to automatic or manual if it's currently set to disabled.
If it's disabled, the D3 Setup.exe will appear to do nothing when ran and the MOP installation will appear to hang on Updating Setup Files. There may be other symptoms but these are two I'm seeing the most right now.
Run as administrator
Make sure you are logged on an administrator account on your computer when installing or patching the game. If the game is not installed using an administrator account, computer permissions can prevent the installation or patch from completing.
Note: We cannot support the game on a non-administrator account.
When running the installer or patch, we recommend opening using the ‘Run as administrator’ feature.
- Right-Click the Diablo III Installer or Diablo III Launcher icon and select “Run as Administrator”.
Security software (Norton, McAfee, ZoneAlarm) is designed to protect your computer from perceived threats. Unfortunately, many security programs are not game-friendly and will frequently perceive a game installation or update as a threat and shut it down.
Disabling or temporarily uninstalling these programs will often permit the Diablo III installation, or in some cases patches, to complete successfully. However, be advised that some security programs will continue to partially run even when they are disabled.
If you are not comfortable modifying your security software settings, we recommend contacting the software publisher for assistance.
The other programs running on your computer could be interfering with Diablo III's installation or patching. We recommended running in Selective Startup mode when attempting to install or patch, since background applications can interfere significantly with these operations.
You can find more information and detailed instructions for this mode on the Closing Background Applications page
Hard Drive errors and Fragmentation
A few bad sectors on your hard drive or a high amount of data fragmentation can result in installation or patching errors. To fix these, follow the instructions on our Running Scandisk and Defrag
Remove Viruses and Spyware
There may be malware on your computer that is interfering with the installation/Patching process. To deal with this possibility, we recommend scanning your hard drive with game-friendly security programs such as Microsoft Security Essentials, Malwarebytes, Avast!, or Kaspersky.
Lastly, memory (RAM) issues can interfere with the game installation/patching. We recommend running memory scan utilities such as the Windows Memory Diagnostic
. Run the extended test suite option, as mild errors on your memory sticks will often not be uncovered with shorter scans.