This may be a long shot, but instead of having FO3 installed, I have backed up all the important files (esp, esm, exe) to my storage drive so that I don't actually need FO3 installed. It's kind of a moot point, but when I run the installer for TTW 2.7.3 with that folder selected as the FO3 origin, it still fails, even though file and folder structure is intact. Point being, what else is the installer looking for when running the script? I have all the proper files in the right place, and yet it still fails automatically. Also, yes this is a legitimate GOTY copy of FO3 purchased through Steam, just to get any talk of piracy out of the way :P
At this point it's not a big deal to install it (just copy the files over to the respective directory and install the rest through Steam), but curious of the BTS.
