Similar problem... brand new MacBook Pros (24 of them). Dual booting Leopard and XP SP3. Trackpad works fine on all but 2 on the XP side...no functionality whatsoever. Finally got it working on both by doing the following:
Go to Add/Remove Programs. Down at the bottom in the Windows Driver Package section, remove Apple Trackpad, Apple Trackpad Enabler and Apple Multitouch (not Apple Multitouch Mouse). Restart the computer. Insert the Mac OS X Leopard install DVD (the one that came with the MacBook). Right click on the DVD and choose Explore. Navigate to D:\Boot Camp\Drivers\Apple. Install AppleTrackpadInstaller and AppleMultiTouchTrackPadInstaller. The trackpad should work after installing the first one; the second one is just for good measure. I think it adds extra functionality. Of course, this is all much easier if you've got a spare mouse to plug-in!
Hope this helps someone out there... as it has taken me the better part of 2 days to get it working.
