Post the actual error message you're getting (can be found in DEBUG files - post in SPOILER tags). It sounds like it could be a virus in bgmain.exe if it can't even recognise that (someone else had this happen recently).
Thanks for the reply. But I kind of got impatient and installed BGT seperately with some minor mods that seemed to work. I´m now well into the game (Cloakwood) and I just don´t have the patience to redo everything (BiG isn´t save game compatable from what I understand). So further assistanse isn´t really needed, still appriciete the effort through.
Did get a bit worried about the virus warning through. Scanned with Kasperskys and ad-adware but didn´t find anything.
Also, the DEBUG files arn´t the same as the BiG world debug text? I kind of did a totall reinstalltion (cleared the directory completely) so I don´t think I have the file any more unfortunately.