A new NVIDIA driver was just released. There's no King's Bounty fix that I see in the release notes, but it's worth trying. I finished the game using 191, so I don't have it installed anymore, so it'd be great if someone else could try these new drivers with the game. If it still happens and you haven't reported the bug to NVIDIA, try to do that.
Link for non-XP 64-bit:
http://www.nvidia.com/object/win7_wi...6.21_whql.html