Bad news for those who've tried the easy fixes and failed:

I tried uninstalling FFXIV, deleting the registry entries, reinstalling, etc. Nothing. It just flat wouldn't update or load properly. I even downloaded a fresh copy of the installer. Finally, I just reinstalled windows, since I like to do it about every 6 months anyways to keep everything running well. Now it works, after a full reinstall of windows and ffxiv. I kept my old FFXIV config folder from My Documents\My Games and it works just fine, so the problem wasn't there for me. Doing this will preserve your UI and stuff, since you won't be able to use the backup option on the loader since the thing won't load.

I have no clue what the problem could be, but it must be a conflict with a system file form another program or something. Either way, talk about frustrating. Enjoy reinstalling everything.