I'm guessing this will help alot of people. I dont know if this has been posted on here before but, owell...
Make sure you close your firewall so it doesnt cause any interference.
1. Open task manager and make sure to kill every agent processes.
2. Delete the C:\ProgramData\Battle.net Folder.
3. Go to Control Panel>Network and internet>Internet Options>Connections tab.
4. Check 'Never Dial a Connection' checkbox and apply.
5. Open your installer, it should work now.
This has been the only successful fix for me.
Hope this helps.
EDIT: I have had this issue with Mist of Pandaria beta also, it is also fixed..