Not sure I can help you. I was running an HPS campaign in Vista 32, and just upgraded my computer and I'm running the same campaign in Vista 64. I don't think your problem is caused by Vista. I am running the BG games in MS Virtual PC 2007 on XP Pro just fine. They won't work at all on Vista. HPS does though.
Here's a tip. DO NOT install your games in Program Files. Create a directory, say, GAMES, and install your HPS games there (heck, install ALL your games there). I bet your problem goes away. Vista protects files and programs in certain directories like Program Files, and some games can't handle that. New games that are specifically Vista compliant would probably work fine.
Lt. General Dirk Gross
XIV Corps/AoC
