The Assets checker is
here. However, it will not check BUFFY_SG12, as it isn't looking for that version. If it did check it, it would fail because of the changes we've made.
Does the standard BUFFY run? Will it load a standard BOTM start file? If not, then something has happened to that as well, and AssetChecker may be able to identify what is wrong.
If BUFFY works then ...
If BUFFY_SG12 loads, check if it runs with a random game (just hit 'Play A Game'). If the interface is still not visible then it probably means the Python scripts are failing to start up when the game begins. There is something wrong with your BtS installation or with the BUFFY_SG12 installation.
Try editing your BtS\CivilizationIV.ini file - the one where you have set autosaves to every turn .... On my XP system it's at C:\Documents and Settings\Alan\My Documents\My Games\Beyond the Sword\CivilizationIV.ini
Open the file in Notepad.
Find the following lines:
; Enable the logging system
LoggingEnabled = 0
Change the zero to a one to read:
; Enable the logging system
LoggingEnabled = 1
Now launch BtS with BUFFY_SG12 loaded and try 'Play a Game' again. When the game starts with no interface, find the PythonErr.log file. On my system it's at:
C:\Documents and Settings\Alan\My Documents\My Games\Beyond the Sword\Logs\PythonErr.log
Make sure you have the right file, as there are several files in that directory with similar names, but only PythonErr.log will have useful diagnostic information. Open it in Notepad and copy its text contents to a post here. Hopefully we'll be able to work out what's wrong with your installation.