In short: Microsoft hates old things. Every time they release a new Windows it tries the best it can to screw up as many old games as it can. I'll be getting Vista next week and I'm terrified what it's going to do to my games collection.
The reason ScummVM exists is because LucasArts adventures older than Full Throttle really struggle with the new platform. ScummVM is an absolutely superb program that's easy to use and setup and ALL your woes will vanish.
If you can't get it to your computer via the Internet (it's a really small download, you could probably use WinZip and put it on two or three floppy disks if necessary!), then it is usually available on magazine coverdiscs.
The latest issue of PCZone has it on their DVD under 'Essential Software', and I bet PCGamer probably does too. Trust us, it will solve all your problems.