I tried everything I could think of: uninstalling old programs, cleaning the registry, updating video drivers (I was convinced it was a video thing), and I scoured the internet. Nothing.
Finally, after using every search combination I could think of, I found the answer on the Adobe Forums (duh). Here's the link.
Quote from user tl_woods:
This is what I did to fix this.
Uninstall all microsoft intellimouse driver versions. Export your registry settings and then search in registry for intellimouse and delete all references.
I dont think this is necessary but thats what I did.
Reinstall with IntelliPoint 5.5...
Its as easy as that.
The problem solved, at last.
And here's the 32-bit version of the driver: