According to Apple’s release notes for the Multi-Touch Trackpad 1.1 update, it’s aimed at users with 13-inch unibody MacBooks released in late 2008 or any MacBook Air model released since late 2008.
Users should download the update while running Windows XP or Vista. When the download is complete, double-click on the AppleBCUpdate.exe file that will appear on the desktop and follow the onscreen installation instructions. If you’re not eligible for the update, a message telling you as much will appear and the update won’t install.
This is the second Multi-Touch Trackpad update for Windows XP and Vista released in the past year; Apple released a similar firmware update in December.