Graev was unable to play Diablo 3 during the beta due to a problem with the launcher giving an error. After pre-ordering tonight and downloading the client to be ready for Tuesday, we ran into the same problem and thought all hope was lost. Graev has Windows Vista and could not get past the “Updating Setup Files” little window that came up. We scoured the support forums for days, Googled, and had no luck until tonight.
Here is the fix that finally worked for Graev:
Go to Control Panel > Administrative Tools > Services
Scroll down and find “Secondary Logon”
Right Click > Properties
Change it from ‘Disabled’ to ‘Automatic’
Run your Diablo 3 setup as administrator and it should quickly go through Updating Setup Files and bring you to an install screen. Congratulations, your game now (hopefully) works! If this solved your issue, spread it around.