If in doubt, blame Windows Vista - it makes even source code work differently. The advantage of Windows was that it was compatible with every software out there (or rather, that every software was built so that it worked on the latest version of Windows), and that every new version of Windows maintained backwards compatibility with old programs; unfortunately, the latest Microsoft programmers, in their enlightenment, broke that backwards compatibility with Vista, so you can no longer assume that old things work the way they used to. Of course, that doesn't mean it isn't an issue with bugs in the game instead of your OS, just that your OS is a possible culprit.
Did you make sure that your friend has the exact same patches and mods installed as you? (I think you should both have them in the same order too, but not as sure on that). Also, do you have BG2 Fixpack installed (I guess that's what you meant by "the fix")?