The original bg1 used directdraw instead of bg2s usage of opengl, and it has a couple bugs in newer versions of windows.
http://bitpatch.com/ddwrapper.html
For the shadow issue I use this directdraw fix. unzip it into your bg1 folder and in the included cfg file change these two parameters to 1 instead of their default 0:
ForceDirectDrawEmulation
NoVideoMemory
Then launch the game as normal. I believe you also need to enable software rendering (maybe also software transparency) in the graphics menu but I am not at my desk to test this.
This should be all you need, he has a weidu mod version of the same software but I don't have any personal experience on how to configure it the way I have, but you can also give it a shot:
http://bitpatch.com/ie_ddrawfix.html