I'm having the same problem as many others on this board are having. No listed action has had any response to what is happening. I've tried downloaded new agent files, changing compatibility modes, redownloading the game, shutting off the firewall/antivirus, adding exceptions to the firewall, deleting the battle.net folder, running agent while the setup is running. You name it, and chances are, I've tried it.
Here's the kicker though: I got the setup files working and reached the launcher yesterday. So imagine my surprise that when I woke up to download the patch this morning before work, and I couldn't reach the launcher.
After fiddling with it some more this afternoon, the setup has reached the point where it will reach anywhere between 60-90% complete, and even saying that it's going to run the Diablo 3 launcher before it will actually lose the progress it had and start over from around 40%.
What can I do about this? This seems like something that should have been fixed in beta. We're less than 24 hours away from launch here in the US and this is more than a little frustrating.