After many trials and tribulations, I have moved this site to WordPress MU (multi-user) on Windows Server 2008 R2, running IIS 7.5, PHP 5.2.5, and MySQL 5.1, all 64-bit (x64)! Now I can run all of my WordPress sites off one database and upgrade them simultaneously.
I highly recommend using this set up if you have to run IIS for other Websites. Your other option with Windows is WAMP (Windows Apache MySQL PHP), which is great if you don’t use IIS or is you don’t mind using a non-standard port for IIS and/or Apache. (Both can’t listen to port 80 at the same time.)
Of course, the standard LAMP (L is for Linux) has long been the best option for WordPress, since it was native to that operating system, but I’m not running any Linux / UNIX servers at the moment.