I've tried installing the game using the pre-downloader (Diablo-III-8370-ENUS-Installer-Downloader) and the newer downloader (Diablo-III-Setup-ENUS), and both result in the same problem, though they did not previously.
Here's the story so far...
Upon my initial installation of Diablo-III-8370-ENUS-Installer-Downloader, the patcher worked fine and the game was fully installed. When I attempted to run, however, it told me my graphics card was not supported. I looked this up on the forums and found that there was an update to the graphics drivers for my RADEON HD 4850, but installing this did not fix the issue. A workaround, however did, so I hit esc and found that the game loaded.
Unsatisfied with the workaround, I downloaded Diablo-III-Setup-ENUS, only to find that while it did not tell me my graphics card was not supported, it instead informed me that agent.exe would not run. After some forum searching and some random experimentation on my part, I found a workaround for this too, but decided I wanted an actually working copy of the game, so I redownloaded Diablo-III-8370-ENUS-Installer-Downloader and tried again.
It gave me no nonsense about my graphics card this time, but instead simply told me that it had failed to launch a critical file.
I tried reinstalling from Diablo-III-Setup-ENUS, and this gave me the exact same error.
My antivirus and firewall are essentially nonfunctional during these installs; I have no other programs running, my drivers are up to date and my computer ran the beta quite smoothly - not to mention I've gotten the program to work before, and can't think of what might have changed in the interim.
Does anyone have any ideas? This has become extremely aggravating.