Sherlock,
I had the same exact problem with Loch Ness as you seem to having and I also am using a Compaq(a 5900Z)with Win98. While my video and sound cards are different than yours, maybe the game just doesn't like Compaqs. I'm grasping at straws here, because the problem is probably something else. But I was never able to finish the game, even with a saved game sent to me by a friend. I'm sorry I can't be of any help, but I thought it's a strange coincidence that we're both running the same brand of computer and both having the same problem.
I hope you have better luck than I did getting past that error message.
Leigh-Anne