1. Click Start > All > Windows Updates, and DL ALL of the updates in there, even if they seem unrelated. You may have a service pack update hidden behind the others. Follow all the steps and don't take shortcuts.
2. Restart your computer (I'm sure many of you have tried this).
3. Go into your files, select the folder (wherever you put it) that contains the Diablo III setup.exe file. Right click the file > select the Compatibility tab > UNCLICK where it says "Run this program in compatibility mode for:" and DO NOT select an option. While in this window you can optionally click the Run as Administrator button, too. Click Apply, then retry the set-up/installation. (THIS ONE worked for me!)
Edit: If your box is unchecked already, check it and find the compatible option for your computer from the drop down box.
4. If your program is stuck on a specific portion, you may have to uninstall and start over again. If it continually gets stuck (mine sometimes said Program Not Responding) it may work after a few minutes, be patient; if it does not work, try doing the setup file from a different Web browser (i.e. Google Chrome, IE, etc.).
If anyone else has any suggestions, or has seen fixes PLEASE post them here. I'm trying to help condense all of the Help advice to one thread to make for easier viewing/assistance. Thanks!