I finally got the idea to ask... I've never been able to fully complete the game because it seems to freeze - right after the magic battle with Mordack, during coversation with Cassima. That colored thing on the right - I guess it's where Mordack put Graham's castle and family after shrinking them - is still moving, I can move the mouse (at this point the hourglass icon), but the conversation message ("After all you've been through, there must be a way!") never seems to end - either by clicking or by itself. So it's not exactly what is usually meant as program freezing, but the effect is the same: I can't see the game ending. Maybe I have a damaged copy? I could try finding another one, play the game again and then see if it works, but maybe anyone has had the same problem and could help me fix it without having to reinstall the game?
I've had the same problem near the ending WAY BACK IN 1995 in the Windows talkie version. It'd sometimes have a SCI crash at that point, too. Forgot what I did to workaround it though.
I've encountered the exact same problem. Couldn't really find anything helpful on the net either - there seems to be a similar problem in the laboratory sequence, with the program aborting with an error message, but the workarounds didn't work for the "freeze" problem you describe. I solved it by going to YouTube and watching the end of the game as a video ... at this point, the game is essentially over, there's just a final sequence with Crispin appearing, saving Graham's family (Alexander is quite smitten with Cassandra, which sets up the plot for KQ6), and teleporting them and the castle back to Daventry.