Try resetting a couple preference files by doing the following:

1. Shut down your computer.
2. Press the power button and immediately press and hold the Command and S keys to start up in Single-User mode.
3. When you see white text appear on the screen, let go of the keys.
4. At the prompt, type mount -uw /
5. Press Return.
6. Then type mv /Library/Preferences/com.apple.loginwindow.plist preferences2.old
7. Press Return.
8. Type mv /Library/Preferences/com.apple.windowserver.plist preferences3.old
9. Press Return
10. Type reboot
11. Press Return. Your Mac should start up normally.

I love how they always tell me to press return. Anyhow.

The computer will boot in single-user mode. It can't find /Library/Preferences/com.apple.loginwindow.plist or the same for *.windowserver.*, and ls confirms that the contents of /Library/Preferences doesn't include any such thing (it does include com.apple.preferences.accounts.plist, but that doesn't quite seem relevant.)

Anyway, any help or suggestions about where to go from here would be mucho appreciado. Even suggestions on where to post this help request are quite welcome.