As far as I can tell, the sengokux.exe and bootmenu is checking for certain install paths for .dlls and .xmls that are probably included in a fresh install of the game, but are missing from the available game files so it thinks the game isn't installed. Probably need the install.exe to get it to run properly. Cri.
Can't seem to find a way to put this into English. Default launches in Japanese and there doesn't seem to be a language option in the menu. Couldn't find (or I couldn't tell) in the game files how to get it in English either.