It was my fault. In the Windows uninstall utility, you get asked which components you want to remove. If you expand "Tor" and "Vidalia", there's a separate checkbox under each for "Application Data", which removes the Tor and Vidalia directories from %homepath%\Application Data, respectively. I didn't expand these options in my hasty uninstall, not thinking about my keyfiles, and they're checked by default for a "complete" uninstall. Oops.
I consider this a "bug". Uninstall should leave your keyfiles behind unless the user explicitly confirms it.
-- Moritz Bartl GPG 0xED2E9B44 http://www.torservers.net/