All known troubleshooting techniques have failed to solve my (and possibly others') issue:

When trying to re-install, whether from DVD or downloaded client, the game WILL NOT complete the update of the Agent files. No matter what—I have tried everything people have listed, but to no avail. It has never created the proper files for me. I had to receive my updated Agent files from a friend to be able to move along to the next issue: The Launcher.

First, "Updating Tools 0%", and that's that. It makes no headway to ever update. I tried the workarounds and it began updating all 14,474 files. After reaching 100%, and getting the, "Your game is fully updated" message, I would click launch, and nothing happens. The play button grays, and the message changes to "Launching...", but that's it. It just hangs.

If I close the launcher and let the Agent file close and then open the launcher again, the process repeats, including the entire updating procedure. That's it. Nothing has helped.

I thought perhaps it was my firewall (even though it's set to accept all Diablo 3 related files), so I turned it off, rebooted, and tried again. Same issues—clearly that's not the problem.

I'm also running on Windows XP, but it is fully updated.