Uninstall the game. Use the uninstall on the CD, and the Add/Remove Programs function of the OS(WIN). Even when you do this, sometimes the Registry enties don't get removed. When the registry entries aren't removed, the CP still thinks the program is installed. Remove the Registry enties that pertain to BG2---WARNING-- if you do not know how to do this, OR have a program that will do it for you, don't attempt this, you could do CP crashing damage.
Now, clean up the ole CP by running a Scandisk and a Defrag.
Re-install BG2(full install).
Install patches and mods in the proper order. The proper order is listed in a sticky thread by Icelus, on this board. BTW, if you have ToB installed, or are planning to install it,,,,install it NOW, THEN install patches and mods. Only use patches and fixes that are marked ToB.
This should fix the problem.
Here is a link to a registry editing tool. The tool has a free trial period..
http://www.macecraft.com/
There are free registry tools out there. I'll post a site when I find one.