Stuck on "Updating setup files..." when installing Diablo 3?

After a reasonable hiatus from Blizzard games, I have decided to reinstall Diablo 3. I used to have it installed and working on this machine and removed my previous install using Add/Remove Programs. However, now when I try to reinstall the game, the Diablo 3 Setup starts 'Updating setup files...' and then hangs;

I have had a quick google for ideas on how to fix it which seems to bring up a bunch of posts when the server loads were really high last year, nevertheless I have been through the list of suggestions, making sure that secondary login is enabled, running the installer as administrator, etc.

After leaving the installer for ~30 minutes, it finally brings up an error stating it could not run the Blizzard agent;

This gives me an error code - BLZPTS00007 - which gives me a few more google hits including a seemingly official guide on how to troubleshoot agent issues on Battle.net, however performing the steps listed under 'Advanced Troubleshooting' still hasn't resolved my problem.

How can I get past "Updating setup files..." when reinstalling Diablo 3, and what is error BLZPTS0007?

For whatever reason, Internet Explorer (which I never use) was set to "work offline". For whatever reason, with Internet Explorer set to work offline, the Diablo 3 installer was failing to do anything other than time out.

I disabled "work offline" by unchecking "Work Offline" in the file menu (available by pressing the ALT key in newer versions of Windows/Internet Explorer);

Once this option was disabled, everything worked first time.