I finally got it sorted. I had all the IIS6-compatibility options enabled under 'Programs and Features'. I was running Internet Explorer as an administrator. But still I was getting a 500.0 error when I tried to go to the Virtual Server 2005 R2 admin page. Tonight I decided I wasn't going to let this one rest.
Eventually I got it fixed by setting the virtual directory to use Basic Authentication instead of Windows Authentication. Go figure.