Well, after getting infection with a Trojan horse, I had to use my system restore CD/DVDs from Toshiba and rebuild my laptop to "out of the box" again.
Three days of backing up files, digging out old CDs, downloading updated versions of free software, and reinstalling everything...and guess what? Calibre installs just fine now.
God knows what was going on with my system, but it'll happily display Calibre fonts correctly for me now.
I'm so happy!
I just wish I knew what the problem was, so I can avoid it in the future.